Zebra Danio became bloated

Question
I had just completed a water change and saw one of my Danios was floating on the surface.  He was still breathing and bloated.  A minute later he stopped breathing.  Now I have a second Danio that is just skimming the surface very slowly.  The remaining 3 Danios are fine as is every other fish in the tank.  

NH4 0ppm
NO2 0ppm
N03 >10ppm

Do you have any idea what could have caused this?  Is it possible they took on air from the bubbles of the water being added?  

Answer
Hi Kevin:  It could be a number of things.  Was the fish ok prior to the water change?  did you use chemicals during the water change?  did the bloated fish look as though his scales were protruding from his body?  A toxin other then normal water chemicals could have caused his reaction as well as a disease called dropsy.  If the other fish doesn't calm down and go about life as normal I would do another small water change and check the filtration.  It sounds like to me there may have been something wrong with the original fish and the excitement from the water change was too much for him.  The bloating concerns me because it sounds much like Dropsy.  If it was Dropsy then that is actually better news then dealing with an unknown because dropsy is rarely contagious.  Keep me posted... dave