newt with big belly

Question
Our newt has a really big belly. It just swelled up and he hasn't been eating much. His belly floats while the rest of him sinks into the water. He is really weak and not moving much at all. Do you have any idea what is wrong with him and what we can do to help him?  

Answer
Well, are we sure its a HE and not a she would actually b my first question.  The next would be what species are we talking about?  What is the water temp at? How often is the water cleaned? Is there gravel in the tank?  Lets start with these first then we can move on from there.