Koi sickness - bent tail, air gasping

[Boz]

I have two 1 foot long, 25 year old koi in a 55 gallon aquarium. Recently one of them has been acting sick. He sits on the bottom of the aquarium with gills pumping, then occasionally tries to make it to the surface to mouth breath. When he swims, it is as if his tail and rear section are not moving normally and will dart wildly to try to reach the surface and will return to the bottom sometimes on his back, struggling to turn over. He also seems to have some trouble with his swim bladder. He is not eating either. The other koi is doing fine and seems concerned for her friend. The ph,akalinity,nitrites,temp are all in spec. I have partially changed the water often,raised the salt level, temp is steady @75 deg, have tried all types of otc remedies, put in extra air bubblers, don't want to give up because of the "old age" factor. Thanks - Boz