Make sure that these requirements are met before introducing your fish to the tank. Always make sure that the fish you buy are well fed. They should look plump and have no missing scales or be scarred.
Active fish are usually in good health.
If the fish seems to be panting or if their fins are clamping, avoid these. If you take them home in a plastic bag, leave the fish in the bag and place the bag in the aquarium. Let it float there for thirty minutes then open the bag and introduce some of the tank water. Let it float for another fifteen minutes or so. This will help your new fish become accustom to the temperature and the makeup of the tank water.