

My fish has this strange thing in its left eye, what could it be? Should I take it to the vet?
His other eye is fine, I'm worried that this eye might be affected. Can it be treated at home, or should we take him to the vet?
I don't know exactly how old it is since I rescued it over a year ago along with another fish, and we also don't know exactly if they are koi or a long goldfish.