I've had fish on their sides starting to float, turned on the uv skimmer and had them perk up in no time.
I've noticed, if you have sand, it may look clean but it's full of nasty bacteria that would kill you never mind some defenseless little fish.
Starting to gather things for my 70 gal. and next time.... crushed coral all the way. (Theres nothing grosser than the red multi-legged blobs of some kind of a disgusting worm that oozes from your sand randomly picking at whatever it can find) BLICK!!!!
Lynn