Heck man if you have a lot of filtration (I run 2 large canisters on my 75) and a good water supply, you can take all the fish out, quarantine them in large bowls/containers, drain the tank to the gravel, completely flip the gravel (getting all the crud off), then fill the tank up with the correct temp water and put the fish back in. Do not touch the filters though (that will disturb the bio).
It's a lot of work (all in all should take you about 2-3 hours) but it might save those fish (if your water is as bad as your describing).
What kind of filtration are you running?