Chapter VII. Cyanobacteria control by NO3 limitation

"They say the definition of madness is doing the same thing and expecting a different result." 
                                                                                                Howlin' Pelle Almqvist

There are suggestions on the forums that cyano problems can be caused by nitrates being limiting. The idea is that because some species of cyano can 'fix' inorganic nitrogen (N) and convert it to organic nitrates, they have an advantage over other organisms that can't fix N when nitrates are low. But, given that I had no idea if my cyano was able to fix N,  I was in no mood for such thinking. So I thought I would try knocking the cyano out with antibiotics and then dosing with potassium but no nitrates or trace elements. I thought maybe adding nitrates had been part of the problem because I had noticed something strange about my tank. Levels of nitrate decreased between water changes and often became undetectable. This went against everything I had read on the forums. Nitrates were suposed to increase, it was one of the reasons to do a water change. Where was the nitrate going? Given that the fastest growing species in the tank was cyano, this suggested to me that the cyano was using up a lot of the nitrate I was adding, and the nitrate from the nitrifying bacteria in the bio-filter, and the nitrate from the decomposition of fs and dead plants etc. It didn't seem likely that denitrifying bacteria (bacteria that convert organic nitrate to inorganic N) were responsible for the missing nitrate because my understanding at the time was that the anaerobic denitrifying bacteria (bacteria that can't tolerate oxygen) couldn't function in the oxygenated gravel of a UGF. Here is a summary of the N cycle I found.

Adapted from: Mills, D. The Marine Aquarium. Salamander Books LTD.
8 Blenhein Ct., Brewery Rd. London N79NT; 1987.

I stopped adding trace elements because I had read that they, especially iron, can cause cyano problems. I hoped that making the tank more nitrate limited might have an impact on the cyano. At the least I thought it would give me a good run at cleaning the gravel. After a major cyano removal session in week 50 I dosed again with antibiotics. As before things went well to begin with. The cyano died and the tank became dominated by algae and plants.

Week 51. Cyano free but for how long?

I was doing weekly 37% water changes and adding K2SO4. I haven't made a note of exactly when the cyano came back, but I think it was getting serious again within six weeks.
So it seemed that attempting to control the levels of nitrate and phosphate did not effect the growth of the cyano. I'm pretty sure cyano will use added nutrients if they're there, but it didn't seem to depend on them. I am not convinced that the idea that nitrates and phosphates effect or cause cyano problems, comes directly from peoples experiences of fish keeping. I think it comes from the environmental sciences. When I started searching the internet for information about cyano I found a lot of scientific studies of algal blooms in lakes and coastal waters. It seems there is no doubt that nutrient run-off from agricultural land causes cyano blooms in nature. Phosphate is often cited as the main limiting factor for these blooms, and they have been reduced by managing phosphate inputs. One thing that strikes me about these studies is that the blooms are seasonal. I was six weeks into year two of my cyano bloom and it was not seasonal. Maybe I was missing something simple.