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Treating Cyanobacteria (blue green algae) with Erythromycin

 Age of Aquariums > Aquarium Articles


A couple of months ago I noticed large amounts of dark bluish algae begin to appear out of nowhere. I quickly learned that I had blue green algae which is actually a bacterium. Great! I thought, now I had nasty smelling bacteria covering all my plants and rocks. At first I tried cleaning the rocks and plants but to no avail, I just ended up with my hands smelling of it; then it would simply grow back. To understand what it was I researched it and came up with this...

  • It is one of the oldest photosynthetic prokaryotes and is believed to have started to produce the oxygen in our atmosphere;

  • It is everywhere in the ground, water and atmosphere, many aquariums get infected by the spores it releases into the air;

  • It can "fix" nitrogen from the air (not relying on nitrates in the aquarium), so even if you have 0 nitrates in the aquarium it will not inhibit growth;

  • It is a bacterium, so it is susceptible to antibiotics, especially erythromycin;

  • Some companies have tried to market this in capsule form as a dietary supplement, even though some species of cyanobacteria are poisonous: they produce neurotoxins, hepatotoxins, cytotoxins and endotoxins.

How do I know if I have blue green algae?
The common signs I have found are:
- a bad smell that you and everyone in the room will notice;
- a thick covering of a strange bluish algae that is coating everything;
- a drop of (or no levels of) nitrates in the aquarium.

How to treat it
The most effective treatment I have found is the antibiotic in Maracyn® called erythromycin, which is sold under the brand name Mardel® this is a plant safe antibiotic that will quickly eradicate the bacteria. This antibiotic is usually used to treat Fin and Tail Rot, Popeye, and body fungus. Maracyn isn't cheap, I had to pay $16 dollars for it (but it's worth the price). Interestingly enough, the price tag was missing and it was the most expensive antibiotic; I doubt it was coincidence.

The steps I took

  1. Day one, after a lot of research, I buy the medicine and start the treatment. Even though it says that it's safe on filter bacteria, I would take out the cartridge for an hour. It is unclear whether most filter bacteria are gram- or gram+. The treatment calls for 1 tablet for every 40 liters. Please follow your medicine's dosing directions so you don't take away its effectiveness, but adding too little can lead you to making a resistant super strain. This is a one time solution, repeatedly doing the treatment could also create a super strain.

  2. Repeat and watch for dead cyanobacteria; remove decaying matter with a small water change (monitor and watch nitrates and ammonia).

  3. Repeat treatment, if the bacteria shows little sign of weakness up the dose by 1/3 or 1 more tablet.

  4. Continued with extra tablet.

  5. Last treatment, most algae should now be dead. Siphon it out.

  6. Jeez, you wake up to find your aquarium cloudy and your fish stricken. Don't panic, it's the cloudy water, just do water changes and they'll be fine.

  7. Another small water change.

  8. Today you wake up to find your aquarium water crystal clear, amazing!

  9. A brown and green algae soon repopulates the aquarium where it was before being smothered over by the blue green algae, the smell is gone as well.

I hope this helps in the fight against this bad bacteria. Be aware that I tried this on a tank of 110 liters with relatively hardy fish like barbs and rainbows. There are other brands that have the antibiotic erythromycin as a main ingredient, so I’m sure those will work as well. Happy Fish keeping!

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