Can a fish stomach explode?

Yes, apparently this does happen. From eating too much or eating something wrong. Perhaps others will shed some light on this.

It's a common misconception that overfeeding fish causes some sort of gastro-intestinal damage that leads to their untimely demise. While it's certainly plausible that a fish, for example a particularly gluttonous goldfish, might overeat enough to develop a long-term health risk, the greatest risks of overfeeding are the result of the uneaten food left to rot in the aquarium.