Ziggy and roka mention good things. I don't think "light" vaccuming is bad, like just the top layer. But deep "disturbing" vacuuming is bad.
You mention that a "small amount" of nitrate and phosphates were in the tank before you used the removers. A small amount of nitrates is to be expected in a small tank, but the phosphates surprise me since you're using RO
water. And it doesn't sound like you're overfeeding, which is the other primary source. Have you checked your source RO
water for nitrates and phosphates? Just because it's RO
doesn't mean it's guaranteed pure. LFSs have been known to let their membranes or DI
resins go too long, and that "pure" water might not be so pure.
Water flow is another thing people blame cyano
on. Personally, I think it's a water issue first, and if you have bad flow it just makes it worse. But just to cover everything, what kind of flow do you have? Any additional powerheads in the tank to keep the water moving?
I'm not really seeing any "smoking gun", unless your source water is bad. I'd try to track down where the phosphates are coming from and eliminate that, rather than rely on the phosphate remover. Stepping up your water changes to once every week won't hurt either. When you do the water changes, suck up as much of the cyano
as you can with your siphon. It'll come back, but if you're diligent for several weeks you should be able to get the upper hand.