not sure where to post about fish injuries, i didnt see anything in the disease section that looked appropriate, so im posting for the type of fish

a silver koi of some kind, about six inches long, has a half circle missing from the base of its tail, where the fins meet the meat, like someone took a hole puncher to him. its very red and inflamed around the wound, which is a couple of centimeters in diameter. i know i shouldnt be mixing the fancy golds and common golds and all but it was originally an oversized goldfish tank, and i didnt want to kick anyone out, all is well, except till this injury. the koi is the largest fish in the tank, by far, the rest are about half his size, and ive never witnessed anyone chasing or attacking the koi, i have removed any questionable decorations, i had some sea glass in there, which im ashamed to admit, but it was uber smooth, i could squeeze it in my hand without fear of injury. regardless it was removed. ive been doing frequent water changes and under dosing with aquarium salt and prime as well. this injury is over a week old. its not looking any better or any worse. the fish swims fine. i havent seen anyone nipping at the injury and the tank is right here next to me so it doesn't go long without some watching. the fish does look sad, which is probably more of an indicator of my guilt than it is an actual symptom, but its swimming normally, and eating well. its got to hurt him though every time he moves i imagine, since its at the base of the tail. it goes almost half way up the base. can i catch and treat the fish with some kind of ointment and a fish bandaid? everything i read via google was pretty injury specific, so AC, i turn to you! please dont chew me out if this sounds like something golds would never do, id cut the base of my tail if i thought it helped the fish. if a picture will make all the difference then i can make a point to learn how to post them here for sake of the fish. any input is helpful, thanks in advance!