Based on what you have described, I first thought it might be a difference
in oxygen content, since the elevation difference between the two ponds is
small, then there would not be much of a waterfall to oxygenate the bottom
pond, like you might have from the filter to the top pond. The symptoms
just didn't fit. Since the waterfall from the top pond to the bottom pond
acts as a skimmer for the top pond, then a large amount of debris will
accumulate in the bottom pond compared to the top pond. I had this
situation after I put in my current small pond, which was connected to two
preform ponds. The accumulated debris will undergo anaerobic digestion,
creating hydrogen sulfide gas which would cause symptoms similar to what you
describe. The hydrogen sulfide would be blown off by the water fall from
the main filter and may not be toxic enough in the top pond, but lethal in
the bottom pond. If so, use PP to neutralize the H2S and clean the bottom
of the lower pond.
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html


"Gallandro" wrote in message
om...
I have two large ponds, One pond is higher than the other (only
slightly) and runs into the other pond. There is then a pump that
takes the water through a filter then back to the top pond. My water
have been tested for Amonia, Nitrate and Nitite, and all read zero.
The strange problem is the fish is the bottom pond have started to
Die. They stop eating, stay at the bottom of the pond them come to
the top to dye, twitching and listing as they go. They look very
healthy apart from clamped fins. There are no visible parasites like
flukes or Ich. This is not the first time it has happened. last time
I think I stemmed it by adding salt or it may just be
coincedence(parasite / bacteria medications have no effect) it went
away so I restocked and now 5 months down the line I'm here again.
any ideas ? remember the top pond is never effected ?

MIKE