Ok, yikes. Any thing you have noticed?
Do you have water test results for ammonia, ph, nitrite and nitrates? Could it be water conditions or chemicals accidentally added to tank?
Any changes to tank or fish stocking lately? What was wrong with the fish when they died? How long has tank been set up and is it cycled?
I'd try upping the pwc's now. If water conditions are ok, I would suspect a bacterial infection as first port to check.