Buying fish at Petsmart may be a bad idea. They medicate thier fish because every tank share's the same filter. Fish tend to die in about two weeks from time you take them home. That's from a Petsmart employee near me. Did you add a new fish recently? Are they all new? Disease can be hard to identify. A product called stress coat is valuable for fish in resisting disease, parasites and handeling fish during water changes. Look into that. Old fish food is also very bad.
You say tap water. Is it from a well? If so, compressed carbon dioxide in the water may be a problem. When filling the tank, allow the water to drop in on top and not below the surface of the water. A strong agitation of the water will allow the CO2 to release somewhat. City water? May be high in chlorine and minerals and there are products available for that at the pet shop. If you have water concerns and you dont own a test kit ask a pet shop if they could test your water for you.
When changing water, I make sure the temperature difference is less than five degrees between tank, holding tank and back into tank. If your using a filtration system, you may not have to change the water for very long periods especially if your using a beneficial bacteria in your tank. A healthy tank will provide its own bacteria to keep it clean and one pound of rocks for each gallon of water will give the bacteria a place to reside. Clean water can go cloudy and back to clean and that happens every summer in my ponds. Bacteria needs time to establish itself and it also controls ammonia.
It all seems very technical, but it is very easy once you get the hang of it.
DJK, of the 80 goldfish and 20 Koi class.