1. Since you did not use a water conditioner, chlorine can play a role in killing the fish.
2. The way you introduce new fish into your tank is totally wrong.
Letting the bag float in the tank for 15 minutes does no good. Sure the temperature of the water in the bag can become closer to the water temperature in the tank, but the chemical makeup of the water are still different. You need to add a little tank water into the bag every 15 minutes or so, to let the bag filled with mostly tank water eventually before letting the fish out. Without doing that, your fish can die to the shock from sudden change in water chemistry.
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3. I just went back to read your first post again, and I noticed that you said you were told to use antibiotics and other medication.
Do not use any medication on the first sign of "trouble". Your troubles aren't even caused by any disease or parasites. Using any medication will only make things worse.
4. As being said, your tank wasn't cycled and it couldn't handle the ammonia produced by that many fish. Ammonia poisoning is one of the leading causes for fish to die in new tanks.
