How long did the fish take to get fat like that?
If it was quick (within a week or two), then it could have a cyst in its body.
If it took a couple of months to get that fat, then it oculd be intestinal worms or a tumour.
I would start with deworming the fish and see how things go after that.
Section 3 of the following link has information about treating fish for intestinal worms.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
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it was rather quick it getting fat, probably a cyst. Thanks for your help. If he/she was hurting he/shes not anymore.
