It sounds like you have quite a bit of fish in there, for a 20-gal
, and you may be overstocked. There may be excess nutrients in the water purely from the fish waste, and the best solution might be to remove some of the fish. I think 2 angels would be plenty for this tank, unless they are wee babies, just for starters. The clown loaches potentially reach 9 or 10 inches (but I hear they are slow growers) so you will need to be thinking about moving them, as well. If this is a common pleco it will get huge, as well, and produce a great deal of waste. They sell these very small, so yours might be a tiny one right now, but I just see a potential problem here.