What To Do When Your Reef Tank Has A Bad Case of Cyanobacteria (Slime Algae) and Diatoms

If you plan on keeping a saltwater aquarium for any amount of time, whether it is a reef tank or a fish-only with live rock tank (FOWLR), you are likely to experience an outbreak of cyanobacteria and diatoms. Cyanobacteria or "Cyano" is also referred to as green slime algae (also comes in red and brown). As it's name implies, Cyanobacteria covers rocks and sand with a thin slimy film. And while it is easily removed by siphoning, if the conditions that support it are not improved, then it will quickly return.

Cyanobacteria and diatoms are brought on by the following factors:

- high phosphate (>.5) and silicate
- high nitrates (>25ppm)
- insufficient current/water movement
- high organics
- low alkalinity (<3.0>

Three products that work to treat the symptom (not the cause - see above) are as follows:

- Chemiclean, which will eradicate the cyanobacteria (reef safe - we've used it in our service business for years)

- Granular Ferric Oxide (removes phosphate and silicates) - we prefer Rowaphos, but there are other brands that work as well too. This is great for removing phosphate, although a bit expensive. Remember to focus on lessening the source of input of phosphate into your system (i.e. tapwater and foods). For tapwater we filter with RODI (target is zero TDS). As for the foods, we strongly recomment rinsing your frozen foods prior to feeding. You'd be amazed at the amount of gook (scientific term) that would otherwise go into your tank/system.

- Doxicyclene hyclate - it has been years since I've used this for treating RTN (rapid tissue necrosis) in corals, but we also noticed that it was great for temporarily eradicating cyanobacteria. I say temporary, because this is just treating the symptom and not the cause. As long as the original cause (see above) is still there, the cyano will likely return.

If your type of saltwater tank is a fish-only with live rock (FOWLR), you are always going to have significantly higher phosphates, nitrates, silicates and organics to deal with than you would in a reef tank - even if you had the same number of fish in a reef tank. This is because in FOWLR tanks, there are no (or very few) corals, sponges, coraline algae, clams, filter feeders, etc. to help remove nutrients from the water.

So with a FOWLR tank, you will need to be more aggressive with use of carbon, GFO (granular ferric oxide - removes phosphate) and with the removal of detritus. I strongly recommend "storming the tank" at least twice a month (during normal/ideal conditions), but I would recommend doing it once a month in your current situation.

What I mean by "Storming the tank", is putting a powerhead on a stick/pipe and blast the rock to liberate detritus that has collected in the live rock. And then having some type of mechanical filtration in use to remove the detritus now that it is suspended in the water column.

To do this you can use a canister filter, D.E. filter (diatomacious earth), or even a small powerfilter (hang-on-the-side). Or if you don't have any of these items, you can storm the tank with the powerhead and then siphon it off the surfact of the rock/sand as you do a large water change. With this method I recommend blasting with the powerhead twice during the water change, allowing it to settle out for siphoning between blasting.

So if you end up with an outbreak of cyanobacteria or diatoms in your reef tank or fish-only tank (FOWLR), consider yourself fortunate that it is just cyanobacteria and dyatoms, which are relatively easy to get rid of, and not something more difficult like bryopsis or hair algae. And remember, you are better off preventing than reacting.