That's too bad about the clown. You've probably already done this but first thing to do when you notice a sick fish is test the major water properties. Temperature, Ammonia, Nitrite, Nitrate, Salinity & pH. If you don't have test kits, you'd need to take water sample to fish store for testing. Then make sure your feeding quality food. Brine shrimp is fine but should not be the only or primary food. Pellets, mysis and frozen formula/LRS type foods would be better food for quality nutrition. Prazipro is mainly for flukes and can help with intestinal worms but will not treat bacterial infections or most other parasites. Velvet or Brooklynella can kill a fish very fast and is common in clown fish. Bacterial infections, flukes and ick are usually slower progressing. Posting a picture of the fish under white light with clean glass and all your water parameters will usually get better responses.