[asylumdown]

I'm not sure there's much you can do to prevent it from returning to be honest. It's present in tanks with high nutrients, low nutrients, and no nutrients. Cyano bacteria is often the only organism living on the bottom of oligotrophic lakes (AKA nutrient poor to the point of being hostile to life) in the world. People beat themselves up trying to combat this stuff and the advice on forums is always 'reduce your nutrients', which is advice that sounds good in theory, but in practice may actually be beyond the control of the aquarist. Here's my theory:

1. my tank has a much higher bioload now than at any point in its history, and I'm feeding ALOT. I have a shallow sand bed that's never been vacuumed in any meaningful way, and my rocks are all marco rock. I also dose baking soda for alk, which means that the pH of my tank is on the low side, probably around 8-8.1 (though I never actually test it). Phosphate is known to have a strong affinity for binding to aragonite, and below a pH of about 8.4 or in a scenario where carbonate and calcium are not spontaneously precipitating out of solution, that phosphate is exchangeable with the water column to some degree. My tank had very low phosphates, aggressive phosphate export through GFO and very little input of anything organic for months and months, during which time I had zero problem with cyano. Since the spring however, I've been feeding way more, and in the summer I fell way behind on maintenance. It is my hypothesis that my rocks and sand have 'filled up' with as much phosphate as they can absorb, and are now acting as an exchangeable reservoir of phosphate in the aquarium. I think this is a big reason why people can basically starve their fish to death for months and still not see any improvement in their cyano problems, it's possible to build up a bank of nutrients in an aquarium that will outlast even the most emaciated fish.

2. I use biopellets for nitrate control, which they are extremely effective at doing. Cyano is associated with carbon dosing of all kinds, likely because cyanobacteria in the wild have been shown to be highly efficient scavengers of dissolved organic carbon. There's also been a suggestion that at least some types of cyano (I hesitate to say 'species' because "cyano" is in fact an assemblage or many different families of organisms) can fix nitrogen directly from the atmosphere. If you can utilize organic carbon plus have nitrogen fixation capacities, that would equate to a serious competitive advantage in carbon dosed tanks, which I think is supported anecdotally based on forum posts.

3. Cyano has a tendency to carpet specific areas of rocks and sand. I don't think this is an accident. I think cyano blankets areas of rocks and sand that have lots of bound phosphate that is diffusing back in to the water column. By blanketing those rocks, they basically catch it all before it ever gets a chance to meet a particle of GFO, effectively trapping the phosphate in your system. Other research has shown that cyano is hyper efficient at recycling N and C with virtually no losses of those atoms once they scavenge it (likely because a cyano mat is actually a complete, microscopic ecosystem, another reason they do well in oligotrohic environments), so it's not a stretch to infer they do the same with P. I'm not sure if you've ever carefully peeled away a carpet of cyano, but underneath it and trapped inside it is a tremendous amount of detritus. It actually traps sludge which would otherwise get skimmed out. I hypothesis that the presence of cyano bacteria can hinder attempts to lower the levels of available P (which you probably can't ever accurately test for) in an aquarium.

What I'm doing to try and prevent it from coming back - first I'm killing it, shifting the balance of what's available to what. Over the next couple of months I'm going to be changing my GFO weekly instead of monthly to try and draw down the bound phosphate levels in my rock and sand as much as possible. I've also recently started an aggressive campaign of sand bed disturbance to try and flush out stored organics, fluidizing my sand bed to turn it over completely and release as much garbage as possible. I've done this three times now and it's definitely helped (the cyano grows back much less aggressively/not at all in those places), but since I don't have a sand vacuum it's pretty inefficient so I'm going to get one soon. I've also started aggressively turkey bastering my rocks. Finally, While I am suspicious of them in general, I'm starting a dosing campaign of a bacterial supplement that allegedly helps to outcompete cyano bacteria for the organic carbon released by biopellets, and I've removed one of my auto-feeders that I realized was feeding food that mostly went uneaten.

We'll see if it works. It might not. I might be totally wrong about what's happening, or my rocks/sand might have phosphate in them that won't ever diffuse out to be exported through GFO, but remain available to cyano via some pathway I'm unaware of. I'm toying with the idea of switching to sodium carbonate for alk as it will raise the pH of my tank and make my rocks less likely to release phosphate in general. I do know that with cyano present , I test 0.00ppm phosphate in the water, but for cyano to be there, P must also be at play, and without killing the cyano first, trying to do something about it is like trying to run the wrong way up an escalator. It might also be that I just happened to have picked up a kind of cyano that could grow in any tank that could also support corals, and the only reason I'm having a problem with it is because I 'caught' it like the aquarium equivalent of an STD. In that case, the only way to deal with it would be to kill it.

At the end of the day this will either work, or it won't. I'm not going to feed my fish any less because they don't deserve to be punished for something that's not the least bit their fault, so if this is just the result of having too many large fish, then I'll have to learn to live with it. I do plan to remove at least one of them in the not distant future.