What to do when your reef tank has a severe case of cyanobacteria (slime algae) and diatoms

If you plan to maintain a saltwater aquarium for a period of time, be it a reef tank or a fish only tank with live rock (FOWLR), you are likely to experience an outbreak of cyanobacteria and diatoms. Cyanobacteria or “cyano” are also known as green slime algae (they also come in red and brown). As the name implies, cyanobacteria cover rocks and sand with a thin, slimy film. And while it is easily siphoned off, if the conditions that support it do not improve, it will return quickly.

Cyanobacteria and diatoms are caused by the following factors:

  • – high content of phosphate (> .5) and silicate
  • – high nitrate content (> 25 ppm)
  • – insufficient flow / water movement
  • – high organic content
  • – low alkalinity (<3.0 meq / liter or <150 ppm)
  • – low pH (<8.2)
  • – not enough herbivores / inverts / sand shakers (hermits, turbo snails, nassarius snails, sea cucumbers)
  • – not enough rock cover with corals (corals and algae compete, so tip the balance in your favor by placing a sock to cover 60% of your rock with corals).

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

  1. Chemiclean, which will eradicate cyanobacteria (reef safe – we’ve used it in our service business for years)
  2. Granular Ferric Oxide (removes phosphate and silicates) – We prefer Rowaphos, but there are other brands that work too. This is great for removing phosphate, although it is a bit expensive. Remember to focus on decreasing the source of phosphate entering your system (ie, tap water and food). For tap water, we filter with RODI (the goal is zero TDS). As for food, we highly recommend rinsing frozen food before feeding. You’d be amazed at how much goo (scientific term) would otherwise get into your tank / system.
  3. Doxycyclo Hyclate – It has been years since I used this to treat RTN (Rapid Tissue Necrosis) in corals, but we also noted that it was great for temporarily eradicating cyanobacteria. I say temporary, because this is only treating the symptom and not the cause. As long as the root cause (see above) is still there, cyano is likely to return.

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

Therefore, with a FOWLR tank, you will need to be more aggressive with the use of carbon, GFO (granular ferric oxide that removes phosphate) and with the removal of debris. I highly recommend “raiding the tank” at least twice a month (in normal / ideal conditions), but would recommend doing it once a month in your current situation.

What I mean by “raiding the tank” is putting a power head on a pole / pipe and blasting the rock to release the debris that has accumulated on the live rock. And then have some kind of mechanical filtration in use to remove the debris now that it is suspended in the water column.

To do this, you can use a canister filter, a DE (diatomaceous earth) filter, or even a small power filter (hang on the side). Or if you don’t have any of these, you can assault the tank with the power head then deflect it off the rock / sand surface while doing a big water change. With this method, I recommend blasting the motorized head twice during the water change, allowing it to settle for the siphon 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 lucky that it’s just cyanobacteria and diatoms, which are relatively easy to remove, and not something more difficult. such as briopsis or hair algae. And remember, it is better to be safe than sorry.

Leave a Reply

Your email address will not be published. Required fields are marked *