Is the water cloudy/hazy? Are you offering strong aeration (not just circulation)? You need to have a strong aerator in the tank - a big air stone is best, but running your protein skimmer with the cup off helps. In this case, it isn't the drug, but the solvent that causes the issues - it is a glycol, and bacteria consume it, and when they do, they take in oxygen from the water and give off carbon dioxide. If you don't have good aeration, that will cause problems with the fish.
Also, prazipro is only effective against certain external worm parasites, it is not very effective against internal worms, and does nothing for protozoan or bacterial diseases. It is very possible that your clown has some other issue and is going downhill simply because the prazi isnt the correct treatment.
Jay