If the platy ate normally and was fat and looked gravid (pregnant) but didn't produce any babies in a couple of months, it was probably full of intestinal worms.
If the fish bloated up (got fat) overnight and stopped eating, it had an internal bacterial infection.
Both of these problems will usually cause the fish to do a stringy white poop in addition to the above symptoms.
The following link has information about fish with stringy white poop and how to treat intestinal worms in fish.
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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