After temperature acclimationg by floating the bag did you just relase the fish into the aquarium? You really need to also acclimate the fish to the pH and SG
of the tank, either through a drip acclimation, or by adding about a 1/4 cup of tank water to the bag every 5 minutes for a ½ hour or longer.
Damsels are pretty hardy fish. They are also quite agressive.
The fish you have probably cannot be saved at this point.
Get some test kits for pH, ammonia, nitrite and nitrate to start with. Lots of folks here like the API kits, BUT get yourself a small syringe at the drug store to accurately measure 5 ml of water. The silk screened line on the test vials is generally not accurate.