I would use a quarentine tank. A small plastic bin should suffice, precautions should be taken to keep the tank extremely clean and to maintain perfect water quality, you should also check the fish daily for any signs of disease, lethargy, not eating, loss of color, discoloration, white spots, ragged fins, or anything other abnormal.
I personally slowly acclimate the fish are float it in a large bag to monitor aggression. I also add more cover for the the fish to retreat to. The bag method works better as it almost represents the habitat and tank mates the fish will be with