When I acclimatise fish by floating the bag for 1/2 hour let the temperatures even out before adding 1/2 a cup of water every 15 minutes until the bag is full.
I net the fish out of the bag and avoid letting any of the water in the bag going into the tank, as there is a possibility there may be something bad in the water.
I never trust a fish shop to keep pristine water conditions and would hate to add
ammonia to a tank or diseases. Once the fish has been netted and is in the tank i would chuck the water in the bag and the bag away.
I agree with Dozey a quarantine tank would be good unless he is the only fish in the tank.
Anna