If the fish have only been in the tank for a few minutes, it is not likely that the water parameters are going to read anything. Fish can not foul up the water in a 55g enough to cause an issue without at least a few hours having gone by. All of the water parameters should match your tap water at this point. Few things, mostly a repeat of the last two posts...
-Could be temperature or
ph shock from rapid acclimation. If the
LFS keeps all the fish in 81F water at a 7.0
ph and you put them in 75F water with a 6.0 or 8.0
ph, that can come near close to killing a fish.
-Did anything odd happen to fish on the way to you house? This is not likely but if fish are overpacked in transit bags and get banged around a lot it can cause problems
-Contamination. If you used ANY chemicals to clean the tank or equipment, those can cause issues very quickly. Since this has happened so quick and to a bunch of fish, I would put money on this.
-Non-dechlored water can cause a big problem too. Did you use water conditioner? What kind of conditioner and how much did you use?
Let us know
ASAP
//Update, NM, posted too late
If the water was there for a week, I am doubting that any possible chlorine would be the only cause of death. Even chloramine usually takes a little while to kill fish. Some people just leave their water to sit out for a few days and add it to the tank with no water conditioner. I am thinking there is something else wrong in the water. Even with people dumping fish straight into tap water with no acclimation, usually people do not kill ALL the fish so quickly unless something else is wrong as well. I would not reccomend more fish just yet...