Don, a groundwater pond is always going to seek the level of the water table around it. Evidently, you have a void somewhere underground. Or, obviously a leak leading to a deeper water source. But, since you live there, you know a lot more about this stuff than I do.

I know of no way to identify the leak without draining. And, generally, draining a groundwater pond can be a character builder. You leak might be on the side or the bottom. If you turn off the pump, you're going to find it.

It's not about the fish. It's about the pond. Take care of the pond and the fish will be fine. PB subscriber since before it was in color.

Without a sense of urgency, Nothing ever gets done.

Boy, if I say "sic em", you'd better look for something to bite. Sam Shelley Rancher and Farmer Muleshoe Texas 1892-1985 RIP Grandpa