I'm trying to figure out what happened to one of my fish and whether it's a fish-on-fish injury or a bacterial/parasitic infection. The fish were fine this morning, and I didn't notice any bullying or sickness when feeding. But several hours later, this little guy was floating on his side with parts of his coloration and scales missing as well as what looks like chunks taken out of him. I have koi and goldfish, and they're all roughly the same size at this point, within a couple inches. He's still alive, so I separated him, gave him a bubbler, and added Epsom salt to his water, but I don't know if he's got a chance or whether I should treat the whole pond. I'm about to test the water and will update if it's not normal. Has anyone seen this or does anyone have advice? Thank you!