One of my molly fish has a swollen left gill. It doesnt seem to be bothering him, and its not discolored or anything. But it is a little bit larger than it should be thats for sure. Could sand be caught in there or something? Im assuming its not bacterial cause there's no signs of discoloration (altough one gill fell off, but i thought maybe he rammed against the log or something.) from what I understand parasites are easy to identify in the gills because of mucus.