I went to feed my fish this morning and one of my platys was lying on the bottom on his side and he wouldn't come up for food. I noticed a couple weeks ago he seemed stressed so I measured the water parameters and ammonia and nitrite were 0, and nitrates were about 20, so everything was within a healthy range. I decided to just keep the tank really clean so I've been doing water changes once or twice a week and everything has been where it should be. All of a sudden, this morning he got a lot worse. He hasn't shown any symptoms of anything other than stress and I've been monitoring him pretty closely. Is there anything I can do to help him? I have no idea what it is because there's not really any symptoms so I don't know how to treat it.