I have a very large (8 feet by 12 feet by 6 feet deep) koi fish pond with a waterfall and extensive filtration system I inherited with the house. In it is a very large and older koi (18 inches long the size of a small salmon) and a smaller, younger one, about 4 inches long.
For the last month or so, the larger has been lethargic. Sits towards the bottom most of the time and eats very little. The smaller one is not active either. For the last two weeks, the larger has been sitting near the top, head tilted downward with little movement.
I've used test strips and a better testing kit and the ammonia, nitrates, nitrites, and ph all appear good with the ph resulting slightly above normal. All other results look good.
I have added salt and Melafix in case there's a parasite. Nothing has changed. They are eating a little food; I don't give them too much.
Today the small one has died. Any ideas why or what else I can do? The big one is still lethargic and sitting near the top with his tail higher than the head. Thanks for any help.
