I live in Sarasota, Florida, near Tampa.

I have a 400-500 gallon pond.

(Background of the pond)

It is a cement pond, it was painted and sealed about 12 years ago. It was home to about 12-15 koi, nothing bigger than 9''. Also had goldfish, minnows, and some comets. There was a water lily, and papyrus. Did not have a pump, or filter. They were as is.

Left for vacation, person left in charge to refil the pond daily neglected his responsibility. We then came home to fish carcasses, and a dried up pond.

Since 6 years after our tradgedy, it was just filled for tadpoles, some goldfish and minnows sometimes it was filled all the way, but we wouldn't fill it daily.

About a week and a half ago, we just recently filled it, and 24 hours later put some feeders in it, and some of the feeders started to die. Sometimes the small feeders die just from stress or being puny. We purchased 3 dozen more feeders and placed them in with the same thing happening... one or two would pop up dead every morning.. After a few days the pond seemed fine and we tested the water, turened out fine so we placed some medium sized koi (purchased from a pet store) in there which seemed fine, Later on I purchased 2 large koi (16''-18'').

Total in the pond, are 4 large koi, and a few medium ones and some small koi and small goldfish, and a pleco.

2 of the large koi were very slow, did not eat, and just sat on the bottom. I just recently lost on after 3 days of just resting on the bottom.

I've taken the water daily to many different pet stores and they say the water is perfect.

There is no run off, the cement is 4''-5'' above ground; so highly doubt water would run off. Since the pond used to be fine before, I'm not sure what was going on with the two larger ones.

Everything else is doing well swimming and eating perfectly. Should I seperate this one?

I was assuming it was lack of oxygen, so there is a waterfall, and a tetra pump with air rock. It is more than I had for the last pond.

Is there anything else I can do for this one fish to treat him, it doesn't look like ich. So I'm not sure what else could be wrong with it.