to Reef Sanctuary BigRedBox~
A bit more information is needed as JKReef suggested. If something like this ever happens again (which i hope it doesn't), take lots of pics of the affected fish and keep a log of your water parameters (I do this all the time
). Pictures paint a thousand words and knowing what type of water the fish are swimming, breathing, and drinking goes a looong way to help determine the problem.
You should be doing regular water changes; enough to keep your nitrAtes down below 10ppm. For a fish only tank, it is important to test the salinity (with a refractometer), temperature, PH, nitrAtes on a regular basis -- daily at first, then less frequently once you get to know your tank. Ammonia and Nitrite testing is USUALLY done during teh initial cycle of a tank but should be tested anytime there is a question regarding water quality or a possible sick fish. If you have corals, you should also test for calcium, alkalinity, phosphates and magnesium.
Definately purchase a skimmer; the biggest one you can
. These help keep the water cleaner and remove nasties before they begin to rot/degrade in the water.