I'm trying to figure out what happened to my koi Angel fish. Today I noticed that he has a mark on his side and I'm not sure if it's a bacterial, parasite, or physical damage issue. He's basically the biggest thing in my 75 gallon tank. The next closest in size to him is a 4-5" bristlenose pleco. The other fish are black and white skirted tetras, a Denson barb, 3 mollies, 4 siamese algee eaters, an oto. I'm trying to figure out what I should put in the tank, meds wise, to help, but I'm not sure what is up.
Water parameters:
Nitrates: <10 ppm
Nitrites: 0 ppm
Hardness: 150-300 ppm
Buffer: 180ppm
PH: 7.6 to 8.0
Chlorine: 0 ppm