slowly dying redcap?

Question
I bought two redcaps 2 days ago from a local store. One of them is displaying all the healthy behavior; however, the other one hasn't eaten his food for 2 days and has slow movements and started to float (right-side up) to the surface, slightly tilted. Hes gills and mouth are still moving. I have conditioned the water, set up a filter and air pump, and the other one is doing fine so far. What can I quickly do to help the sickly one?

Answer
Hi Esther;

There are several possibilities but without enough information about your tank I really don't know. Since the sick one was purchased recently I would take it back to the store. Most stores have a guarantee of some kind.

The information I need to be able to help you make sure your other fish will be okay, and possibly figure out what's wrong with the sick one;

What size is your tank?
How big are the fish?
How long has the tank been set up?
What kind of filter system?
What is the water temperature?
Do you use a water conditioner?
What are the ammonia, nitrite and nitrate readings? (it's okay if you don't know these readings, but most local fish stores can do a test for you. It's important.)

Let me know as soon as you can...

At Your Service;
Chris Robbins