What's wrong with my American Bullfrog?

[Crispy42]

Do you have a water test kit like the kinds used for fish aquariums? This will tell you your water quality by showing the amounts of ammonia, nitrites, and nitrates that are in the water from decomposing waste. If you don't have a test kit, you can bring a water sample to Petco or Petsmart and they will test it for free (go to the aquatics section). You might need to increase water changes (even with a filter) to keep these levels down.

Tap water should be find as long as you are adding a water conditioner to remove any chlorine or chloramine from the water. If you haven't been using a product like this then I would definitely recommend getting some to make sure his water is safe for him.

I really wouldn't recommend releasing him, especially in the middle of winter. Plus releasing captive animals can wreak havoc on the natural ecosystem if they aren't natively found there. Even if the species is native to your area, he could be sick and expose wild frogs to illness or parasites that they are not prepared to deal with.

[Thevacantface]

Follow what Cris said, and start conditioning the water immediately, that sounds like it is possibly the issue. As for the quality, same thing, Cris is right. Even if the water looks clean, and the frog hasn't been in it long, it could still be toxic, you should change it every day, or test and treat it every day and change when it gets bad.