There are several things that can cause it from constipation, improper water conditions, nutrition, and wounds. There is no cure for it, but some things can help it like, providing a quarantine tank with shallow water, 1 teaspoon of salt per gallon, thawed out frozen peas for food, and submerge food and allow it to begin to sink when feeding the fish can help.
Sometimes swim bladder can be caused by deformities. These can be corrected by surgery although its not recommended. It may be there food. There to change there food and feed them pease for about a week. This cure worked on my goldfish so I see no reason why it shouldnt on yours. Boil some frozen/fresh (not caned) peas, mush them and remember to skin them as fish can choke on the skins.
I disagree my fish had swim bladder and peas actually cured it. I think, it died a few days later!
Apparently you can't cure it my fish has it at the mo and peas don't work