Sorry to hear that you have another bloom of cyanobacteria. No one knows what causes cyanobacteria to suddenly appear in an aquarium and the proposed reasons you hear are based on studies of floating cyanobacteria not the mat forming variety you are dealing with.
Another difficulty in understanding cyanobacteria is that it just one microorganism population of bacteria and algae of a community of hundreds of microorganism populations and these populations are not stable.
For eaxmple, every time you feed an aquarium, the size of some of the bacterial poulations grow and then decline from predation, death and mechanical removal by the skimmer. The bacterial community likely responds to many other changes in the aquarium. And when one population of bacteria grows or declines, the other bacteria populations are effected by the crowding or freeing up of nutrients and space. Cyanobacteria is part of this dynamic, fluctuating bacterial community. The periodic reappearrance of cyanobacteria blooms may just be part of a periodic fluctuation in the bacterial community.
Because of the complexity of interactions within the bacteria community, no one factor is likely to be the trigger. Worse yet, we don't have the test equipment to measure the factors likely to be involved.
Nutrients are an extremely important factor in controlling the bacterial community and continues to be the focus of most attempts to stop cyanobacteria blooms. Unfortunately, it seems to take a long time time to adjust and rebalance a bacterial community.
While the skimmer is a useful water purification method, it does an incomplete job of removing nutrients. GAC is another good purification technique but we cannot see it working or measure its effect. As a result, I think that we are not using enough GAC and not changing it often enough to thoroughly strip organics. GFO is good at removing phosphate, but if you are not regularly monitoring phosphate, levels can quickly creep up.
An interesting but unproven idea is probiotics, the addition of bacteria to consume nutrients. By adding more bacteria of certain types to the bacterial community, you are shifting the balance. Does it work to curb cyanobacteria? Does it only work by continually adding bacteria?
Or will a bacterial communitiy adjust and become balanced over time anyway without doing anything but waiting? I am going that route. i am dealing with cyanobacteria in a fish only aquarium. I am not considering light or flow as particularly important factors. I am using GFO to keep phosphates undetectable as indicated by the Salifert test. I am using twice the recommended amount of GAC and changing it in half the recommended time. I have the GAC in an up flow reactor to ensure good contact. I am also mechanically removing the slime mats because I find them ugly, although under the microscope they are a fantastic community of organisms. Since starting the use of GAC, the cyanobacteria mats are going green, becoming thinner and growing thin, long hairs. It has been over a month now but I guess it will be another month or two before it disappears. I want to give every remedy I try a good long chance to work, hence the patience. If it is still a nuissance after Christmas, I might see if probiotics has an impact.
I hope that I have provided you with a useful perspective.
Good luck.
