I have had 3 little bristlenose plecos in that tank for about 3-4 weeks. They are kinda tiny and I often don't see them for days, then I might see one or two scamper around. I am wondering if one of them died or got picked on by a larger fish, and his remains caused a bacterial bloom. But this was kinda sudden, and I can't believe a tiny fish like that would cause such a problem in a 90 gallon tank. I have counted my other fish a dozen times to make sure I am not missing one that is causing the cloudiness, but nothing so far.