execution chamber!

I have an outdoor pond - concrete and masonry - - perhaps 4 ft x 10 feet by
a couple of feet deep. It has an effective filtration and circulation
system. Until last year, my critters lived in it in peace and harmony.



Then:



It began to lose water fairly rapidly through a large crack that had
developed in the bottom. I drained the pond and called in my favorite
neighborhood handyman who cured the problem by cleaning out the crack and
filling it with an epoxy compound. The pond no longer loses water - - now
it kills fish. Everything I introduce to the pond DIES. After many months
(and many sacrificial fish) I declare myself thoroughly STUMPED.



Any ideas? I innately suspect that the epoxy has something to do with it,
but the water has been changed and filtered so long that it doesn't seem
plausible.



Any or all suggestions, ideas, hints or cures will be cheerfully
appreciated.