Andy Sager
Aquarium Advice Addict
I'm of the opinion that either it's bacteria or some other fish ripped her very preggers belly open. She's still alive. If she lives til tomorrow I'll begin treatment, otherwise I'm really not thinking she will make it that long.
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There is another possibility, it's coming from within. If the fish gets bound or can't deliver, the fry may be too large to stay inside and are just ripping her insides up. (I couldn't tell from your pic if the wound was internal or external. ) That's the danger sometimes of breeding livebearers that are not "the same." If you breed a smaller female or one that produces smaller fry with a male that produces larger fry, the female cannot always handle the load. If there are too many fry inside, she bursts from within. Guess you'll know tomorrow.