I would be very hesitant to count on bentonite to keep a gravel bottom from leaking. I do not recall anybody on this forum posting positive results about it fixing or even slowing down leaks. I would much prefer a 9 ft deep hole with good clay, well packed, on the bottom and aeration to keep the water viable and the fish healthy.

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