Chapter V. Cyanobacteria control with antibiotics

There is a lot of information on the internet about treating cyano infested tanks with antibiotics. Almost always it is pointed out that such treatments only address the symptoms. If the underlying causes aren't dealt with the cyano will come back. My thinking was that I had dealt with the underlying cause of excess nutrients. What I couldn't understand was why the cyano still grew so well. It was as if cyano, once established, was in some sense self sustaining. I thought that antibiotic treatment might knock-out the cyano to a point where the new conditions in the tank meant that it wouldn't dominate. In week 38 I bought a product called cerpofor aerocol No Blue Algae. I disconnected the under-gravel filter (UGF) riser from the power-head but kept the power-head running with its venturi valve open, removed as much cyano as I could and dosed. Three days later I did a 37% water change and reconnected the UGF. I measured 0.1 ppm nitrite (NO2) at this point but levels soon dropped to zero so it seemed my bio-filter (nitrifying bacteria) survived. All the cyano left in the tank had died and was easily siphoned off. I started 37% water changes every five to six days and dosed with potassium and trace elements. As before I added nitrate when levels dropped below 1 ppm. The great thing was, because I wasn't having to siphon cyano off everything, I could devote the full 20 liters to cleaning fs out of the gravel. I stopped using the Algarde and just stuck the siphon tube into the gravel (with a plastic grill to stop gravel going up the tube). It was amazing how much I could remove. It was a glorious time, I remember green hair algae growing on the bog-wood, it got quite long. Eventually back came the cyano. I haven't made a note of the exact date, but I think it was obviously back by week 42 (four weeks on from the antibiotics). Here is a photo taken before a water change in week 49 (11 weeks on).

There are less plants in the tank because cyano seems to effect them eventually. The Ellodea would grow from one end but die faster from the other thus shortening and eventually going yellow and dying. It's hard to know if my cyano produced toxins which affected the plants or if they used up all the nutrients I was adding and so out-competed the plants (search for cyanobacterial blooms in the environmental sciences literature if you're interested in cyano toxicity). The Amazon swords were coated in cyano, but if you removed the slimy film they looked green and healthy underneath. They stopped growing but didn't die like the Ellodea. Maybe the cyano was toxic to the Ellodea but not the Amazon Swords? One thing was certain, the cyano was dominating my ecosystem again.

Nice shot of the cyano on the bog-wood and some Hygrophila polysperma plants I had added to see if they did any better than the Ellodea (week 49 again).