Spirit's list is dead on. I would like to add Chlorine to the beginning.
Here's a very brief synopsis of what happens with water in a fish tank:
Fresh tap water is added. Most likely, there is something in that water that is not good for the fish. Well water needs to be tested to insure there are no low level toxins present. Municiple tap water has chlorine. Adding unaged or untreated water to a tank that has fish present will most likely result in the death of the fish. A simple, concentrated product called Prime will eliminate any concerns with Chlorine contamination.
Now, we go into the nitrogen cycle:
Fish waste and rotting matter in the tank produces ammonia. Overtime, beneficial bacteria establishes and thrives on the ammonia, which is then converted to nitrites. Bacteria then again "consumes" nitrites, which is converted to nitrates. Nitrates will remain in the tank and continue to build. Fish can tolerate low levels of nitrates without any concerns. Once levels reach 20-40ppm though, your fish will begin to suffer. The only econimical way to remove nitrates is to change out 30-50% of the tank water with fresh water. Now, going clear back to the top of this reply, you start all over.
Typically, this is a weekly process. Most get in a routine to do water changes every 7 days or so. But this really depends on stocking levels, filtration, feeding habits, etc.
The only positive way to know what is going on in your tank is to use a Master Test Kit (around $35 and available most any fish or pet store). I highly recommend getting one.
From your post, I am guessing your fish are dieing from chlorine poisoning. To give the remainders a shot at surviving, I would go immediately to your LFS or Petsmart and get a bottle of Prime. Add one capful for 50 gallons.
Post back with updates.
Good luck!
