<human>: How can I tell if the fish in my aquarium are healthy?
<bot>: One way to tell if your fish are healthy is to observe their behavior. Healthy fish should be active and swimming around, not hiding or sleeping at the bottom of the tank. You may also want to check for any signs of illness such as changes in coloration, drooping fins, or cloudy eyes. Additionally, you can test water quality by checking the pH level and ammonia and nitrite levels.