Swollen belly and gulping

Question
My male guppy has developed a swollen belly and is gulping at the surface of the water. A previous guppy also developed a swollen belly, which I didn't think much of, but we found him dead several days later. We have an air pump and water filter, and we do a partial water change every week. Do you have any ideas as to what could cause this behaviour? Is there anything we can do to help him?

Answer
Charlotte ,                                                  He could have acquired dropsy. It happens where they just can't get rid of enough of the liquids in their bodies. They drink by osmosis and relieve themselves of liquid wastes this way.I would recommend adding aquarium salt to the tank 1 tablespoon. per 5 gal... to prevent this from happening again ... Good Luck, Tina