Uninvited Guests in the Aquarium

Bear enjoying the aquarium

The first part of this ebook is found here.

When we set up an aquarium, we seek to produce a warm, chemically benign, nutritient-rich environment in which our plants and fish thrive. Such a favorable environment is attractive to uninvited guests. At best, these guests merely compete with our regular aquarium inhabitants for nutrients, and may even be beneficial. At worst, they regard our regular tank inhabitants as nutrients. Some can wipe out a previously healthy tank in short order.

  1. The Variety of Biological Interactions
  2. The Tree of Life
  3. Predators
    1. Fish
  4. Parasites
    1. Protozoa
      1. Ich
        1. Diagnosing ich
        2. Treating ich
        3. Preventing ich
      2. Oodinium
        1. Diagnosing oodinium
        2. Treating oodinium
  5. Pests
    1. Algae
      1. Diatoms
      2. Black brush algae
      3. Green hair algae
      4. Green spot algae
      5. A note on algae eaters

The Variety of Biological Interactions

Scientists love classifying things, because it is a way to bring order to their understanding of the beautiful variety of nature. Biologists are particularly fond of classification, and they broadly classify all interactions between living things as short-term or long-term. Short-term interactions include predation and pollination; only the former is of much interest to aquarium keepers. A clown loach feeding on a snail is a predator. For our purposes, so is a clown loach feeding on a prized Amazon sword plant.

Long-term interactions, or symbioses (singular symbiosis), are classified by whether one or both species involved are helped, harmed, or largely unaffected by the relationship. This can be diagrammed.


Wikimedia Commons

Aquarium keepers are undestandably less interested in the welfare of uninvited guests than of their regular tank inhabitants. They are also less interested in relationships that are largely neutral with respect to their regular inhabitants. This means that mutualism and commensalism, where our regular aquarium inhabitants benefit from the presence of guests, and parasitism, amensalism and competition, where our regular aquarium inhabitants are made worse off by uninvited guests, are of greatest interest.

I'll therefore look at uninvited guests from the perspective of whether they are predators on our regular inhabitants, are parasites on our regular inhabitants, are competitive pests, are harmless, or are actually beneficial. Predators and parasites are two of the relationships formally recognized by biologists. Pests are either amensals or competitors, depending on whether the guest is significantly hamred by the regular tank inhabitants. Harmless guests are mostly neutral or commensal, depending on whether the uninvited guest gains significant benefits from being around the regular inhabitants. Beneficial guests are either commensal or mutualistic, depending on whether the guest benefits significantly from the presence of our regular aquarium inhabitants.

Much of what I present here is purely for its scientific interest. However, there is obvious practical value in knowing how to rid our aquariums of predators, parasites, and pests, and how to encourage beneficial guests to flourish.

The Tree of Life

The regular inhabitants of most planted aquariums come from just a few branches of the tree of life. Fish are a single branch of the chordates (vertebrates and their close kin), which are a branch of bilaterians (animals with bilateral body plans), which are a branch of the animal kingdom, which falls within domain Eukaryota. Snails are gastropod molluscs and shrimp are crustacean arthropods, which are also bilaterians. A few planted aquariums also include amphibians, which are relatively close relatives of fish, and crabs, close relatives of shrimp. Our plants are mostly tracheophytes (vascular plants), though we keep some mosses (bryophytes) and (less commonly) red or green algae.

Uninvited guests range much more widely across the tree of life. This is now known to be much more complicated than the ancient division into animals and plants. Biologists recognized that many microscopic organisms were neither truly animal or vegetable, based on their cellular structures, even before we gained the ability to sequence the entire genome of different species. Genetic sequencing has confirmed suspected relationships, and uncovered unsuspected relationships, by revealing common genes between different species.

The more fundamental a gene is to carrying out life processes, the more likely it is to have settled into its current form early in the history of life, and the less likely it is to have changed much since then. Such gene sequences are described as highly conserved. Arguably the most fundamental of all genes are those that dictate how genes themselves are translated into proteins. These include ribosomal RNA, which makes up the individual protein factories. These are very highly conserved. Biologists confirmed soon after rapid sequencing became possible that bacteria have very similar ribosomal RNA, but it is significantly different among what where then called archaebacteria, which more closely resembled ribosomal RNA of eukaryotes. This led to the recognition of the three main divisions or domains of life, consisting of the bacteria, the archaea, and the eukaryotes.

Bacteria are very simple single-celled organisms in which, for the most part, everything takes place in a single cellular compartment. Archaea resemble bacteria (and were long classified as such) but have very distinctive biochemistry that is actually closer to that of the eukaryotes. Eukaryotes have more complicated cells with multiple compartments (including, at a minimum, a nucleus to hold genetic information) and include plants, fungi, animals, and ourselves, as well as complex single-celled forms of life that are able to move about, traditionally identified as protozoa, and simple photosynthetic organisms, identified as algae. Although the protozoa and algae are now understood to include many groups that are not closely related, and there is significant overlap between the two groups, these terms are still in common use among aquarium keepers and others.

Here is a simplified diagram of the tree of life as currently understoold. I have eliminated some levels that are still controversial or do not distinguish living organisms of interest to aquarium keepers. Click to enlarge.

        of life


Predators are guests whose short-term interaction with our regular tank inhabitants is to kill and eat them. Many of these are not actually uninvited, but are organisms we have deliberately introduced into our tanks. We then discover to our sorrow that they have made themselves unwelcome by developing a taste for our other fish, invertebrates, or plants.


It's really no surprise that certain species of fish are not good choices for a planted aquarium.  The following tropical fish have bad reputations:

All of these either have a tendency to chew up plants, or are burrowers that uproot them.

An additional difficulty with plecostomus and clown loaches is that, under ideal conditions, they can grow to be very large.


Parasites are organisms that satisfy their nutritional needs by attaching themselves to a larger organisms and feasting on its tissues. They are distinct from predators, which prey on smaller organisms that are typically eaten whole, and from mutualists, which "pay rent" for taking nutrients from a larger organism by giving important benefits in return.  A clown loach feasting on a snail is a predator; a nitrogen-fixing bacteria living in a plant root is a mutualist; and a ciliate eating into the skin of a fish is a parasite.


The great majority of protozoa in our aquariums, though uninvited, are not unwelcome. They are the microscopic counterparts of snails and algae-eating fish, feasting on overgrowths of bacteria and algae and thereby keeping them in check. Only a few species are parasitic on plants or fish. Unfortunately, these can cause a lot of trouble.


Ich-infested fish

Perhaps the the worst of the parasitic protozoa is Ichthyophthirius multifiliis, "fish louse with many children." This is a ciliate, a member of a large family of protozoa that share many traits and, presumably, a common ancestor. This most distinctive trait is the presence of cilia, hairlike organs that move in a coordinated way to propel the ciliate through the water or to draw water past the ciliate. Other characteristics shared by most cilia are a mouthlike organelle called a cytostome, through which the ciliate takes in food particles (typically smaller microorganisms), and a division of its genetic material between a macronucleus and a micronucleus. The micronucleus is the ultimate set of blueprints for the ciliate, and it is duplicated during asexual reproduction and swaps genes from another ciliate during sexual reproduction. The DNA blueprints in the micronucleus are copied numerous times in the macronucleus, which is where DNA is copied to RNA that will then serve as the template for protein synthesis.

Most ciliates are harmless or even beneficial, but a few species are parasitic, and ich (as we call it for short) is one of the worst.  Most experienced aquarium keepers have had to deal with one or more bouts of ich. Fortunately, it can be prevented by appropriate quarantine, and an infestation that gets through quarantine can be eradicated with suitable chemical treatment.

Ich is the fish equivalent of smallpox. It covers the fish with small white spots (about the size of a grain of salt) but the worst damage is inside the gills, where it is not as visible. Fish heavily infested with ich die from oxygen starvation from the smothering effect of the gill infestation, or from loss of electrolytes through their damaged skins.  Ich has no other host than fish and does not survive long on its own, but it spreads rapidly and kills most of its victims if left untreated. Fish that survive ich develop significant resistance to future infestations from the same strain, but a practical ich vaccine for fish has yet to be developed.

Ich has a fairly elaborate life cycle. The spots you see on your fish are the feeding stage of the parasite, known as the trophont.

Ich trophonts. Via Wikimedia Commons.

The trophont is a single large cell, covered with a coat of cilia and with a very large horseshoe-shaped macronucleus. The white spot consists both of the trophont and of the inflamed tissues of the fish that surround it. By the time a white spot is visible, the trophont has already been feasting on the fish for some time and is close to maturity.

A mature trophont is up to a millimeter in size. When it reaches maturity, it burrows back out of the fish and drops to the bottom of the tank, where it transforms itself into a tomont. A tomont is sometimes described as a cyst, but it it not a true protozoan cyst: It cannot survive being dried out, and it cannot remain dormant for more than a short time. A tomont is sticky and will adhere to whatever solid object it comes in contact with, including a fish net dragged through the aquarium. It is a reproductive phase of the organism, in which the giant cell divides into several hundred therodonts, also known as tomites.

Once the tomont has finished dividing into therodonts, the therodonts burrow out of the tomont wall and become free-swimming organisms.  These resemble other ciliates but apparently have none of the usual organelles for capturing and digesting smaller microorganisms. They must find a host fish within about 12 hours or they will starve. Those that find a host burrow into its skin or gill membranes, transform into juvenile trophonts, and begin feasting on skin and blood cells.

This life cycle explains why ich is so explosively dangerous and why it must be treated the way it is. The complete cycle from trophont to trophont takes about seven days at a typical tank temperature of 25 C (77 F). It is much slower at colder temperatures (eight weeks at 6 C (43 F) and faster at higher temperatures. Above 30 C (86 F), the tomonts of most wild strains are unable to replicate at all, but it is now clear that heat-resistant strains have evolved at fisheries that can withstand temperatures higher than the fish themselves can long tolerate.  Each tomont produces hundreds of offspring, so a single infested fish can spread the infestation throughout the aquarium in just a few days.

Diagnosing ich

The first symptons of ich are often behavioral, appearing even before the first white spots are noticeable. These behavioral clues are likely due both to initial infestation of the gills (which is not outwardly visible) and to the fish's reaction to infestation, which is similar to the general feeling of malaise (prodrome) we experience at the start of a bad bout of influenza or other infectious disease. Infested fish will often gather at the surface of the water, sometimes over the heater or in another warm part of the tank, and become listless and lose interest in food. Close inspection will then reveal a few characteristic white spots. Fish may "flash", that is, scrape themselves on rocks or other sharp surfaces in the tank, apparently in an attempt to dislodge the parasites. Some species of fish, such as Siamese algae eaters, will occasionally flash even when healthy, but any increase in flashing should be considered a warning sign and the fish should be closely inspected.

The definitive diagnosis at commercial fisheries is made by netting out one of the fish and taking a scraping of its skin to search for tomonts under the microscope. Home aquarium keepers may not have a microscope and are understandably loath to stress an already sick fish in this manner, and, for us, the appearance of white spots the size of salt grains on the fish is considered definitive. At this point, the fish are already heavily infested and likely to die if treatment is not undertaken immediately.

Treating ich

The trophont and tomont stages of the ich life cycle have considerable resistance to any treatment that will not kill the fish as well. As a result, all treatment approaches other than heat treatment are aimed against the free-swimming therodonts. If these can be exterminated, the life cycle is broken. With no new organisms burrowing into their skins and gills, the fish can begin to heal as the tomonts drop off, encyst, and produce therodonts that are destroyed by treatment as fast as they emerge.

Heat treatment. This was long a standby of aquarium keepers who were keeping fish able to withstand the high temperatures required (30 C or 86 F). For example, discus keepers often kept their fish permanently at 86F and almost never experienced an ich infestation. Unfortunately, there are now credible reports, supported by my own experience, of strains of ich that can withstand such temperatures and cannot be eradicated by heat treatment alone.

Nevertheless, the first step in treating ich is almost always to raise the tank temperature as high as the fish can comfortably stand for a couple of weeks. This will be somewhere in the ballpark of 28C (82 F) for most tropical fish. The idea is to speed up the ich life cycle, which makes treatment more effective by pushing the organisms into the therodont stage that is vulnerable to medications.

Malachite green and formaldehyde. The standard chemical treatment for ich is a combination of malachite green and formaldehyde.  Malachite green is a drug that neither contains malachite (or any other copper compound) nor is really green (it is closer to sky blue in color).

Malachite green

Formaldehyde is a very simple, rather toxic molecule famous as the preservative for specimens being dissected in biology classes.


Malachite green plus formaldehyde is highly effective at killing ich therodonts, as well as a number of other protozoa and bacteria. Either chemical can be effective by itself against ich, but the combination is synergistic, meaning it is much more effective than either medication alone. It is speculated that formaldehyde attacks cell membranes, which allows better penetration of malachite green to the vital structures within.

Standard dosing is 0.05 ppm malachite green and 15 ppm formaldehyde. However, I do not recommend formulating your own mixture; ich is not something to screw around with, and neither are formaldehyde and malachite green, both of which are suspected carcinogens. I generally refrain from specific product endorsements (or criticisms), but I have gotten excellent results with Kordon's Rid-Ich Plus (I do not recommend their "organic" product) against strains that were resistant to both heat treatment and malachite green alone. Use exactly as directed, which includes a daily 25% water change before administering the dose for the day. Since there is evidence that therodonts preferentially break out in the early morning hours, the best time for treatment is late evening; but if you have confirmed an ich infestation, do not wait to make the first treatment. The water change is best done with a gravel vacuum, which will suck up some of the tomonts and therodonts at the bottom of the tank. Vegetation and decorations should be vacuumed as well.

Malachite green plus formaldehyde seems to have little effect on nitrifying bacteria, but it is wise to monitor ammonium and nitrite during ich treatment to be sure. It is also the received wisdom that the mixture is irritating to clown loaches, catfish, and other scaleless fish. It is sometimes recommended that half the usual dosage be used on tanks containing such fish, but my experience is that Kordon's is fine at normal doses with such fish so long as water changes are made before each dose, and I think the fish have a better chance against the medication than they do against the ich. Since such fish tend to be bottom dwellers, where the heaviest infestation of therodonts is present, they tend to suffer worst from an outbreak, and aquarium keepers may be confounding this greater infestation with increased sensitivity to the medication used to treat it.

Some plants may suffer from the treatment. I have found that vallisneria tend to slow their growth and staurogyne may suffer as well. Other species seem to tolerate it well.

Malachite green is easily adsorbed by activated carbon and is degraded by bright light. You must remove any activated carbon from your filter before beginning treatment, and, if possible, you should dim the aquarium lights until treatment is complete. Once the ich is eradicated, you should reinstall your carbon filters (or install them if, as recommended earlier, you are avoiding them in your planted tank.) This will remove the last traces of the drug. 

Treatment is complete when the fish return to their normal behavior and show no white spots for at least three days.

Copper. Copper salts, such as copper sulfate, are highly toxic to ich at levels of 0.15 to 0.3 ppm. Unfortunately, such levels are highly toxic to any invertebrates in the tank, and toxicity varies with water hardness in a way that is hard to control.

Methylene blue. This can be effective, but it is usually not more effective than malachite green, and methylene blue has a much greater tendency to destroy nitrifying bacteria in the biological filter.

Quinine. Quinine is effective against nonresistant strains of ich, and since it is rarely used any more, most strains will be nonresistant. The difficulty is coming by it; it is no longer sold over the counter for cramps in the U.S., as it once was, and it is not a normal part of the pharmacopoeia of most veterinarian clinics.  I have not found it in fish shops in years.

Metronidazole.  Some fish shops carry metronidazole, which is effective against a number of bacterial infections as well as ich. However, it is expensive, and it should not be tried unless a strain of ich emerges that is genuinely resistant to malachite green and formaldehyde.

Salt bath. Wild strains of ich cannot withstand salt concentrations of 5 grams per liter, while most fish can. The fish may even benefit from the reduced osmotic stress in brackish water. However, plants cannot stand salt at this concentration, so this is a useful strategy primarily for a hospital or quarantine tank. And if a single fish shows signs of ich, the entire tank must be regarded as infested. Moving the entire fish population of a display tank into a hospital tank is not practical for most of us, though it is an excellent idea if it is. Treatment then requires less medication, and more treatment options are open. The display tank need not be medicated if every fish is removed, since the infestation will be starved out after a few days. The hospital tank must not only have enough room for the entire stock of fish, but it must also be cycled adequately for such a large stock.

Preventing ich

Prevention is vastly better than treatment. Ich has no host other than fish and it has no true carrier state. Thus ich can only come in to our aquariums as an unwelcome hitchhiker on plants or animals added to the tank. 

There are three approaches to ensuring plants do not bring ich tomonts into an aquarium. The first is to obtain plants from sources we can be certain are free of ich. This generally means plants grown in the absence of any fish whatsoever. This will include the increasingly popular tissue cultured aquarium plants. 

The second approach is to disinfect plants obtained from unreliable sources. I have not found reliable information on what is required to destroy ich infestations, but the tomonts of its marine equivalent, Cryptocaryon, have proven vulnerable to any of the following:

The question is which of these the plants themselves are likely to survive. I suspect plants have considerable resistance to a temperature of 40 C, but so may some heat-resistant strains of ich. Benzalkonium chloride is readily available on the Internet and is mild enough to be used in consumer products, including as a preservative in eye drops, but is dangerous in higher concentrations. It seems worth testing whether aquatic plants can tolerate it at 100 ppm. The procedure would be to make up the proper concentration of benzalkonium chloride in water, immerse the plants for a full hour, and then rinse thoroughly before moving into the tank.

Chlorine is also readily available as household bleach, but this tends to contain other ingredients and to vary in concentration. The aquarium keeper may be better off buying calcium hypochlorite sold for use in pools, which also has dosing information for achieving the desired concentration. Pure calcium hypochlorite is about 50% chlorine, so 120 mg/l will produce a chlorine concentration of 60 mg/l. If household bleach must be used, it must be pure bleach. Bleach sold in the U.S. must show the concentration of calcium hypochlorite, from which the dose can be calculated, but may not list all the inert ingredients. Be aware that 60 ppm chlorine is fifteen times the maximum value recommended for drinking water and six times the level considered unsafe for swimming any length of time in a pool; I am skeptical that many plants can stand an exposure of 24 hours at this concentration. But if you try this, the plants should be rinsed in water dosed with a dechlorinator before being moved into the display tank.

Drying is obviously something aquarium plants cannot survive. However, it is worth knowing that nets or other aquarium equipment exposed to tomonts will be safe to use in uninfected tanks when thoroughly dried for a full day.

Since most methods of disinfection seem likely to kill the plants, we are left with the third approach, quarantine. This is also the only approach that is reliable for fish. In the case of plants, if they are held in a quarantine aquarium containing no fish at 27 C (80 F) for a full week, any tomonts will have hatched and the therodonts starved out. The plants should then be safe to move to the display tank. This seems like the safest reliable method for ensuring plants from questionable sources do not carry ich into the tank.

Quarantine of fish is the only reliable way to prevent new fish spreading ich to the display tank. Even the best fish shops are constantly bringing in fish from supply chains that are constantly struggling to control ich infestation, and it is all too easy to purchase a fish that has already been infected but does not yet show symptoms. I personally resolved to never skip quarantine after battling two ich infestations in one year.

Quarantine must be long enough that even fish with a light starting infestation and partial resistance will have time to show telltale symptoms. Two weeks is  the minimum for fish that come in appearing healthy and never show any signs of infestation. The conservative recommendation is that freshwater fish be quarantined at 27C (80 F) for a full month. If ich does develop and is successfully treated, the fish must be kept in quarantine a full month from the last signs of infestation before moving them into the display tank. This implies that the serious aquarium keeper must have a quarantine tank large enough to comfortably hold any fish he plans to purchase for weeks at a time.

Invertebrates, such as snails, can still carry tomonts on their shells if they have been kept in the same tank as fish. Since they cannot serve as hosts for the infestation, a week of quarantine should be adequate for them.


Oodinium. Via Wikimedia Commons.

Oodinium is remarkably similar to ich in its symptoms and life cycle, in spite of being only distantly related. It is a dinoflagellate, a member of a family of single-cell organisms traditionally regarded as algae. Modern molecular biology puts the dinoflagellates in the Alveolata, a clade (branch of the tree of life) alongside such unlikely relatives as ciliates and the malaria parasite, and distinct from other algae.

Dinoflagellates have a pair of flagella and a distinctive set of outer membranes, most are photosynthetic, but most also consume smaller microorganisms. Their chloroplasts are unusual in having an additional outer membrane. Most are free-living marine organisms, though some live in fresh water.

Oodinium (actually Piscinoodinium pillulare) has chloroplasts and carries out photosynthesis, but it also attaches to the skin of fish and feeds on their skin and blood cells. Like ich, it has a life cycle consisting of a reproductive palmella, a host-seeking dinospore, and a parasitic trophont. The trophonts are much smaller than ick and have a brownish to gold color, so that fish appear to be covered with brown velvet. This gives the parasite its common name of velvet disease.

The condition seems to be less common than ich but is no less dangerous.

Diagnosing oodinium

Fish will show the same behavioral symptoms as for ich. The trophonts are tiny and are best seen by dimming the room lights and shining a flashlight directly on the fish. This will show an iridescent golden film on the fish.

Treating oodinium

The same treatment protocols are effective for oodinium as for ich. There is some tendency for aquarium keepers to use copper, if possible, for treatment, since Oodinium is very sensitive to copper. However, malachite green plus formalin can also be effective, and it is much less toxic to invertebrates. Salt baths are also effective when practical; the trophonts will reportedly actually detach from the fish in saline water and can be rinsed away. Acriflavine, which is less commonly used for ich, is effective against oodinium.

The chief distinction in treatment is that oodinium gets significant nutrients via photosynthesis. The infestation can thus be slowed down and more easily eradicated if the tank is kept as dim as possible.


Pests are free-living organisms that are unattractive, compete for nutrients, and can overrun the tank. They include fish and invertebrates that develop a taste for other plants, fish, or invertebrates.


Algae are a diverse group of single-celled and colonial organisms that carry out photosynthesis. Only the green algae are closely related to green plants. They are normal tank inhabitants but become unsightly if their growth goes out of control and they take over the aquarium. Dealing with algae is less a case of eradicating them than of controlling their growth.

Aquarium keepers tend to identify algae by their form as seen with the naked eye. This is understandable, but some forms of growth are characteristic of algae from very different branches of the tree of life.



Diatoms are single-celled algae that are not closely related to green plants or most other algae. They are part of a large group called Harosa or Sar, which also includes Oodinium and Ichthyophthirius.This group and the Archaeplastida, the red and green algae and green plants, plus some other small groups, make up the bikonts, which include almost all the descendants of the first eukaryote to carry out photosynthesis. Many of the bikonts have since lost their photosynthetic ability, including the ciliates.

The distinctive feature of diatoms is that they surround themselves with a silica shell. This is beautiful under a microscope. Diatoms are an important part of many ecosystems, and their photosynthesis produces 20% to 50% of the Earth's oxygen production via photosynthesis. There are both freshwater and marine diatoms.

Diatoms often appear in new aquariums as a brown film or crust that feels gritty to the touch but is fairly easily wiped away. Diatoms thrive on the silica from fresh substrate and in the lack of competition for nutrients. They almost always disappear in time, in part because of competition for nutrients from plants and other algae, and in part because they are feasted on by everything from algae-eating fish to microscopic amoebas.  When diatoms persist in a mature tank, this is typically an indication of high levels of silica in the water supply. Silica can be diluted with deionized water or removed by ion exchange, but ion exchange media that remove silica tend to remove phosphate needed for plant growth as well. Diatoms thrive on the yellow-green light that green plants absorb poorly, so the use of grow lights will help plants compete with diatoms.

Black brush algae

Black brush algae Black brush
              algae under microscope

These are among the more unwelcome pests in an aquarium. Black brush algae, also sometimes called beard algae, is actually a form of red algae, such as Audouinella. It comes in all colors from dark green to nearly black. It can overrun a tank and it tends to grow over green plants and smother them.

The convention wisdom is that black brush algae does best in a tank that is deficient in carbon dioxide or other nutrients. It can absorb yellow-green light and so the use of grow lights will help plants compete with it. However, it is persistent, and it attaches itself so strongly to any surface (including plant leaves) that it is very difficult to remove mechanically. If your green plants are all thriving, they seem to suppress the growth of black brush algae; but if the plants are struggling, black brush algae can quickly take over.

Black brush algae is vulnerable to the glutaraldehyde in "liquid carbon" products, and the algicidal properties of glutaraldehyde probably account for more of the benefits sometimes seen from "liquid carbon" than any carbon enrichment. Particularly persistent patches of black brush algae can be treated by squirting part of the glutaraldehyde dose directly on the patch with a syringe. Hydrogen peroxide can also be used to treat isolated patches, and it leaves no residue in the tank. (This can be a good or a bad thing, depending on whether you want residual glutaraldehyde to continue working on patches you miss or cannot directly treat.)

Siamese algae eaters will nibble on black brush algae, though they are unlikely to bring it under control on their own.

Green hair algae

Green brush algae Vaucheria
              under microscope

These resemble black brush algae, but are green or yellow-green algae and so are dark to bright green in color. They adhere tightly to surfaces (driftwood seems particularly favored) and have a texture almost like steel wool. A number of not necessarily closely rated species of algae fit this description, and precise identification can be difficult even with a microscope. My best guess for the variety growing in my tank is Vaucheria, a yellow-green alga.

These algae are almost impossible to completely eradicate from a planted tank, and many aquarium keepers choose to make their peace with them. A modest amount of green hair algae on a piece of driftwood can in fact be fairly attractive. They like almost precisely the same conditions as green plants, including lighting, and they seem impervious to glutaraldehyde. Siamese algae eaters will sometimes pick at them but they are not a preferred snack. They do seem to endure low nutrient levels better than green plants, so keeping the aquarium well fertilized for the light level, along with removing as much as possible by hand, is the best hope for control.

Green spot algae

These are also always present in a healthy aquarium and can never be entirely eradicated. They form green spots on just about every surface in the tank. They come from many families of algae, but are typically green algae such as Coleochaete that are very close relatives of green plants. Those that colonize the glass can be removed with an algae scraper, but those that colonize plant leaves are not so easily removed. The best control, other than overall balanced tank conditions, are algae eaters such as snails, otocinclus, ancistrus, or Siamese algae eaters. All enjoy young colonies of green spot algae, though they are unlikely to keep the aquarium walls completely green or prevent a slow buildup of green film on the slowest growing plants. Otocinclus are particularly useful for removing some green algae from plant leaves, while ancistrus are a good alternative to larger plecostomus for keeping hard surfaces cleaner.

A note on algae eaters

The usual algae eaters rarely consume much black brush or green hair algae. Even Siamese algae eaters are unlikely to keep heavy growths of these forms under control. This does not mean that an aquarium should not have some algae eaters. I consider Malaysian trumpet snails and otocinclus as near mandatory and larger snails (mystery or nerite) and an ancistrus as highly desirable. Malaysian trumpet snails help keep the substrate aerated and clean up debris,while otocinclus are unexcelled for removing softer algae from plant leaves.

It is not unexpected that an aquarium will eventually acquire a population of algae that are unappetizing to whatever algae eaters have been placed in the tank. This is a natural consequence of the selective pressure applied by the algae eaters. They are eradicating algae types that are not visible in the tank.

Although Siamese algae eaters are among the best for consuming the tougher forms of algae, I have grown a bit skeptical of them. They much prefer whatever else is available to eat in the tank and will feed heavily on the tougher algae only if kept nearly starved, which is impractical in a tank containing any non-algae eaters. They also grow rather larger than most novice aquarium keepers realize, often to 6" or better, but their appetites do not grow in step. Instead, they dominate competition for flake food or whatever other food supply is being offered the other fish.

Lately some species of gara, such as Garra gotyla gotyla, have become available on the market and are claimed to be excellent black bear algae eaters that do not grow larger than 3" or so.  Information is still lacking, but some sites describe these as omnivores, which makes me skeptical. I also have no information on whether they are inclined to eat green plants as well as algae. I will follow their career with great interest, to coin a phrase.

It is also important to properly subsidize algae eaters of all types. A population of otocinclus that is large enough to scrub an aquarium clean of softer algae is a population of otocinclus that is in danger of starvation if not offered supplemental food. This can take the form of algae wafers or of blanched vegetables (such as peas, leafy greens, or sweet potato) periodically placed in the tank as a treat. These should not be supplied to excess so as to completely spoil the appetites of the algae eaters, but should be sufficient to ensure they have enough to eat. An oto with a round fat belly is a happy oto; if the otos are round bellies but are continuing to scrub surfaces in the tank, then you have struck the right balance.

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