Aquarium Cyanobacteria

An Aquarist's Guide to Blue-Green "Algae"

Your tank is running well, except for a light dusting of an intense green algae on parts of the glass. All of the sudden, sheets of bluish-green stuff are spreading across the decor. Still, it's just an algae outbreak. Easily delt with. You step up your Water changes and manually remove the algae. Day after day, however, it comes back. You buy a cleaning crew of shrimp, snails, and/or fish known to eat algae, and yet nothing seems to change. Worse, you think your fish are getting sluggish. You check the fish disease charts, and they don't seem to fit any of the profiles. They sometimes hover near the top of the tank, at other times the bottom. They never seem to gasp for breath, but always are languid. The more tender inhabitants may even be dying without apparent reason. You bring out the big guns. You go out and buy a bottle of algaecide. And still, nothing happens, other than the death of any inverts and live plants that happen to live in the tank.

This is not an uncommon occurrence, nor are these actions unreasonable, given the information that most aquarists have. For all intents and purposes, this seems to be some sort of algae outbreak. Even its name, "blue-green algae" says so. However, it isn't an algae. It's actually a strain of bacteria known as cyanobacteria (its name derived from its vivid color, which would be beautiful if it didn't signal so much trouble for an aquarium). In addition to blue-green, cyanobacteria can be black or even red.

What Is Cyanobacteria?

Some cyanobacteria is beneficial, being an important part of the nitrogen cycle. Spirulina, which is hailed as a "superfood," being rich in all of the amino acids, as well as other important nutrients, is a form of cyanobacteria. Others, however, produce various forms of neurotoxins, hepatotoxins, cytotoxins, and endotoxins. All forms of cyanobacteria seem to be somewhere between plant and bacteria. They have a gel-like cell wall (cell walls are usually reserved for plants) and are fed partially by photosynthesis. They also possess bacterial traits. Some are free-floating, some form threads, or sheets, or even hollow spheres. Thankfully, it seems that most of the harmful cyanobacteria take the form of brilliant sheets, making it easy to identify.

A few things contribute to the beginning of a cyanobacteria outbreak. Too much light, too much phospate, and general poor water quality can begin an outbreak. Introducing plants that have not been quarantined can bring on an outbreak in a seemingly healthy tank. Once it has begun, however, cyanobacteria can be much harder to get rid of than an algae outbreak.

What to Do?

If you are facing an outbreak of cyanobacteria, there are a couple of options for treating the tank:

Antibiotics for Cyanobacteria

The first is a fairly simple remedy. Dosing the tank with an antibiotic will kill off the infestation pretty quickly. This has several downsides that offset its ease and speed. The first is that every use of antibiotics has the potential to create a strain of antibiotic-resistant bacteria. You may get rid of the cyanobacteria only to find that your tank has been infested with a strain of fish TB that doesn't respond to the antibiotics. The second is that some fish and inverts don't deal well with certain antibiotics. The third is that dosing a tank with antibiotics is a good way to completely un-cycle the tank.

A good way to at least partially bypass the loss of your tank's cycle is to pull the media out of your filter and store it in another tank. If you don't have another tank, you can seal it in a bag with some tank water and keep it in the refrigerator. After you have set aside the filter media, treat the tank. Once treatment is done, run fresh activated carbon in the tank for an hour, then replace the filter media. This should leave your tank with a significant portion of its nitrifying bacteria intact.

Starve the Little Buggers

The second method, while more work and time intensive, has no real negative effects on the tank itself. In fact, it is, in general, good for the fish. The first thing to do is to thoroughly clean every surface of the tank. Second, step up water changes to lower the phosphorous levels in the tank. If your water supply normally contains phosphates, you may want to invest in some phosphorous-removing filter media. Third, kill the lights for several weeks. Fourth, feed your fish less. Most fish food contains phosphorous, which ends up in the water, feeding the cyanobacteria. Combined, these actions should starve the cyanobacteria out of the tank. This procedure takes time, of course, which is its greatest downside.

During either procedure, it is a good idea to remove the bacteria as it appears. You can often get it with the vacuum if you lightly scrape at the sheets with the edge of the vacuum attachment.

Preventing Cyanobacteria

As with most tank problems, the best way to deal with a cyanobacteria outbreak is to not let it happen. Frequent water changes will help keep phosphorous levels down. Quarantining all new livestock and plants for several weeks will minimize the chance of introducing a virulent strain of cyanobacteria to your tank. It is far easier to treat a quarantine tank, or even just break it down, than it is to do so to a fully set up aquarium. Feed your fish only what they can eat in a minute's time to further limit phosphorous as well as waste products.

Although it is a pain to get rid of, if you know about cyanobacteria, and if you react properly to it, this does not have to be a tank killer. The main reason it is so difficult is that many aquarists don't know how to deal with the issue. Once that hurdle is behind you, it should be no problem to deal with an outbreak of cyanobacteria, if one does happen.

About the Author

Sam started keeping fish when I inherited a tank from a friend's girlfriend. Have since purchased three other tanks, each with an entirely different setup and type of fish.

Cyanobacteria - Red Slime Algae pic courtesy Agsansoo. Thanks Andy.