I got a few comet goldfish a while ago, and I think one of them is sick. I recently set up another 40 gallon tank below the tank my flowerhorn is in, and in it I have two comet goldfish, a lionhead goldfish, and a small koi. I think the tank might still be cycling, but I think it's almost fully cycled. The smaller comet goldfish is acting sick, it almost looks like he has fin rot, but all the other fish are acting fine. He is sitting in the corner with his dorsal fin down, and it looks like he might have fin rot on his tail fin, but his pectoral fins are fine. I'm going to do a water change on the tank, but I'd like to avoid putting medication in the tank, as all the other fish are health. Is there anything else I should do?