After all the tests and dead goldfish, I believe that I have
determined a possible cause. When adding a large trash bag filled
with ice to cool the pond, I caused considerable waves in the pond. I
noticed that there was a small puddle next to the pond which moved in
sync with the pond water disturbance. I believe that there is a leak
in the pond somewhere (difficult to tell with this black preformed
liner) which is in equilibrium with water which has collected under
the pond. The water under the pond must have some toxic substance in
it which is contaminating the pond. This conclusion makes sense to me
since water taken directly from the auto-refill does not kill fish,
but water coming from the auto-refill system into a freshly cleaned
pond liner does quickly kill fish.
I think you have probably found the problem. I had preformed ponds for my
first two ponds, hooked together with a short stream. The ponds cracked on
the third season. My ponds were located above the septic system leach
field, and I was having a lot of problems that I think may have come from
the exchange of water between the pond and leach field. If this is the
case, then you could be getting significant amounts of hydrogen sulfide in
the water which is very toxic to the fish, and is not one of the usual tests
that we perform. Check for the leaks.
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RichToyBox
http://www.geocities.com/richtoybox/pondintro.html