Fatty liver or egg impaction, cyctic kidneys. There is not a lot that the average hobbyist can do about this complaint .. clearly there the problem is internal and therefore requires a koi specialist or a good informed veterinary surgeon.

By all means you can try giving the infected koi a Salt bath using 1.5 to 2 ounces of salt per gallon of pond water .. but please ensure that you place at least one air stone in the bowl and keep your eye on the fish whilst it is in this saline solution .. and if it begins to show signs of distress, like laying over on its side, then return the fish to the main pond .. If this seems to help then you can treat the koi twice per week in this manner.