Sounds like he's doing what a fish does. Swims and jumps, Water tests good. Those answers go together very well. Now if you would really like to seek a more detailed answer give more detail information. I know that sounds harsh and like I'm being a smart-... but seriously, to really be able to help we need to start somewhere. Water tests "Good" tells us almost nothing. What has been tested? PH, GH, KH, Ammonia, nitrite, nitrate, salinity, heavy metals, oxygen? yes every single one and more can be tested. You are asking for help, we don't know the pond size, the water parameters, no pictures to refrence. The more you know, the more you can determine. That's just the begining, Once the water is tested, Do you have a microscope? Don't treat until you know with certainty what your treating. Most parasites ( Which can very easily be a possibility, but just one and then what kind of parasite out of the hundreds that are known!) A good microscope cost about the same as a nice two year old koi, Is the investment worth it? I say yes, knowing what is wrong and how to fix is the most challenging but also most rewarding part of the hobby. We all want to keep our swimming friends for many years. When I read a post "If" I can help I always try to. I can't help here as really it could be one of so many things and it could be, just a fish that's doing what he feels like doing. The more information you can provide, the more helpful others can be when trying to assist.
