Thank you all for your thoughts. The pond has always been refilled
using an auto-refill valve put in by the original landscaper who
installed the pond. The fact that fish lived in a bowl of the same
treated tap-water that is used to fill the pond suggests to me that
the problem does not lie in the municipal water source. It is
possible that it has something to do with the auto-refill I suppose.
As far as the pond liner, it is one of the preformed liners from home
depot, so I doubt that that could be the problem. As far as the
pumps, there is a waterfall pump (Beckett waterfall pump) and a
Beckett medium fountain pump. As far as the water parameters,
recently the temp has been around 83 degrees. The pH measured at
about 8.0. As I previously mentioned the ammonia level was tested by
the LFS, who reported that is was a little high (I am afraid I dont
remember the number or the units) but not deadly. The fact that this
pond and all equipment, and rocks in it supported fish successfully
for the past two years, is what is so confusing and frustrating for
me. I will try to test for some of the other parameter mentioned.