We have a pond and we have numerous fantails/comets that we raised from squirts and we also had two very special koi that we raised from 2-3". The koi are now 2' and we have had them for 6 or more years. Anyways 2 days ago one was not swimming, and yesterday although still breathing was floating in his side. The other one, my favorite was not acting right either. By they time I got home last night the one was still floating on his side but breathing (it was killing me to see this) and the other was not far behind doing the same. I filled up the bathtub with cold tap and put in the cholorine neutralizer, started an air pump and brought the two fish inside in the hope of saving them (but I really new better). Well, they did not make it and we are devastated. Feels just like loosing a cat or a dog or any other pet you have had for awhile. We just can't figure out what happen to them. No signs of disease or trauma. And, not one fish but two, very odd. Since the pond was over wintering some of the water readings were off but I don't think that would be the case.
The fantails/comets are all fine. Anyways, my heart is heavy today. Tomorrow will be their burial.

Sorry to hear about your fish, that was a very impressive carp, 2 feet is pretty big. I think you would have to get an autopsy done. Sometimes it's worth doing if you've got lots of expensive fish, after all, if your fish has got a contagious disease, it would be a good idea to know about it. Not trying to worry you.

sorry to hear about your loss.6 years is a long time to have a pet pass without really knowing what went wrong.