Sick koi?

My koi has been acting strange. At feeding time he acts like he can't see sometimes, running into the sides of the pond. He seems listless and like he can't keep his balance. He ends up vertical, nose down, and seems to really fight to stay balanced. The other fish bump into him and he just floats out of the way, or is getting knocked out of the way. I recently introduced 2 smaller koi about a month ago. They don't really bother him. He is the largest fish in the pond. The comet I have has become really aggressive though, coming up underneath of the koi and bumping him. I think he will be getting a new home soon. Could my koi be stressed out? He has no visible signs of sores or eye disease. Thanks!

Serena

Generally, Koi fish are very sensitive types of fish, which is why there are some substances that must be avoided (either in their food or on its habitat). However, there are also some certain chemicals that must be diluted into their habitat. Another sensitivity trait, of which Koi fish are often subjected to, is the chemicals that are added into its food.