Bloating; Not Good....

[NutMutt]

I have noticed that two of our fish (one black Molly and one Swordtail) have started to bloat-up on us. It seems like they are on their way to becoming ballons and "pop". I noticed the Blacky Molly was at the corner of the tank on the surface for some now. Breathing in air. Which I have read is not a good sign.

Anybody have any idea as to was might be causing this?