Most posters appear to be focusing in on the auto-refill system.
Although I find it hard to understand how some type of toxic substance
could get in and persist in there after so much water has gone through
the system, I will try some of the suggestions on testing this (i.e.
filling the pond with tap water from another source, and testing
goldfish in a bowl filled with auto-refill water). Once again the
auto-refill comes off an outside hose water outlet (supplied by the
city water supply).

Thanks,
Timothy