They say , "Every GREAT CHEF HAS BURNED A FEW MEALS " so your in good company , and not the worst fish keeper .How long before they die? Do you quarantine a new fish before you put it into your display ? what kind of fish are you buying ? " ,you didn't say . What is near the tank ?{cleaners ? maybe} Don't give up , I couldn't say how many of my fish have died do to mismanagement[ though not in a long while ] . At least your trying to get answers .You don't really learn by not making mistakes . Is there somewhere else besides petco to get your water tested[ste2k3 metioned 2 ] ?A second opinion , maybe 3 if you are spending alot of money . Find out how close your tanks water chemestry is to the stores tanks ,