I'm no expert on this stuff, so please take what I write in the shrugging, palms-up spirit in which it's intended.
This is a long thread, and if you read through enough of it, you'll realize people have had a wide variety of results doing a wide variety of things to fight dinoflagellates:
- Blackouts to interrupt photosynthesis
- Manual removal - siphoning rocks and sand, and/or using filter socks
- Starving them out through nutrient reduction - refugiums and/or algae scrubbers
- Starving them out via nutrient elevation - helping other algae grow in order to outcompete them
- "Dinocides" like Algae X
Even people with the same type of dinos (e.g. Ostreopsis) can have completely different results with the same approach. How can that be? It almost doesn't make any logical sense at all.
My gut is that because these things are heterotrophic, they have a wider range of acceptable conditions (light, pH, nutrient levels, trace element levels, and so on) that they can live within than your average nuisance algae. When the combination of those various levels in your system hits a certain threshold, there are too few nutrients for the other algaes but enough for the dinos, so they suddenly have little to no competition and blossom out of control. So
either raising
or lowering those levels enough can take you back out of that dino zone in one direction or the other and solve the problem. The question then becomes how to stay at that new level, since your system obviously gravitated to where was for a reason. And that new level could come with its own set of ancillary issues (other nuisance algaes, poor coral growth, etc.).
Trying to beat these things seems an awful lot like fighting cancer. We try different approaches based on the symptoms we see and parameters we can measure, but there are things going on at this level about which we don't have enough info and knowledge to fight intelligently. Who knows how many variant strains of Ostreopsis there are, or how each strain's specific nutrient requirements differ? I don't know that it's possible to have enough detailed info to plot a course of action that you know will work in a given tank, so all we can do is try different things and pay attention to the results.
In my case, the Algae X and reduced light seem to have brought the dino population way down for the moment - but I seriously doubt it's permanent because I haven't changed anything else in my system. So for longterm control, I hope to go in the direction of very low nutrients since I'm an SPS freak. I'm going to be religious about my filter socks and will probably start filtering down to 50 μm. I plan to start dosing BioDigest/Bioptim and I'm also setting up an algae scrubber to pull out nutrients. I'll probably starting feeding more and may dose an iron supplement along the way to help maintain algae growth in the scrubber.
I don't know if any of this will work, but it's the extent of my plan at this point. If I don't feel like the tank is back on track in 90-120 days of this struggle, I might try changing salts to change the trace element balance. Beyond that are only the drastic measures like nuking the tank and starting over or getting out entirely.
I'm sure none of this is news to you, but if it helps you decide what to try next then at least you're taking steps. In that light I hope it helps.