More likely there's ammonia in the water poisoning him. If you have no filter(which means your tank is not cycled), then the ammonia produced by the fish poison it slowly, they get sick.
Best thing to do now is do a 100% water change, make sure the water is dechlorinated and mix it with some boiling water to bring the temperature to the same the fish is in now(test with your hand). This will remove the toxins from the water for now but they quickly build up again so you need to do constant water changes. It is stressful for fish to do 100% water change, but in this case if you have no filter in the tank it's the best thing to do. Then every day take out 50% of the water and change it with dechlorinated water and twice a week do a 100% water changes to keep the ammonia to minimum.
Best thing in the future is to get a filter as well in the tank. If you do have a filter, it's probably not cycled yet.
Read this about cycling and different ways to cycle a tank:
http
/www.fishforums.net/index.php?/topic/277264-beginners-resource-center/