When you're a fish and you eat something extra, the fat isn't stored between your skin and muscle as it is with us. It's inside the body cavity, around the organs. This can interfere with the workings of their bodies rather quickly.
I've kept fish for close to 6 decades, and I still enjoy feeding them. When I was new at this, my fish got treats and they went for them. Now, I'm boring and give no extras but one. If I swat a mosquito, it goes into a fishtank and someone gets a high fibre extra. Otherwise, I feed sparingly no matter how much the fish beg. They don't need food as we do, as they don't have to run an internal, energy burning temperature regulation system. They're old blooded, and they can get by on much less (hence no mechanism for safely storing fats).
This was a really good question because I think we've all gone there and wanted to give treats to the animals in our care. Or, if I'm honest, we still want to give them treats. You have to channel that into improved daily nutrition.