Is the hose hooked up to your sink or at least a faucet that you can adjust the water temps? If you are all of a sudden adding cold water to the tank that could be why your fish are dying on you. Their body's cannot adjust to temperature changes so fast like that.
Also, if you are not adding a water conditioner like Prime each and every time you do a water change, that could be killing your fish. The Ammonia, Chlorine in the water needs the conditioner to take it out.
How many fish have you lost?