I'm unfamiliar with "Cotswold" sandstone, but if it's a true sandstone, I
don't think you have a problem. Most rock will not release any "stuff"
(unless it's shale, which will release an oily substance which is NOT good!)
that will drive your pH too high; many of us use limestone to buffer the
pond and keep pH in balance. If it's been there a while without any problem
to the pond, it's OK.

Lee

"P Verstege" wrote in message
om...
Hello. I have a new 8' x 17' x 3' PVC-lined pond populated by 72
assorted goldfish/shubunkins and 3 koi. Around the outside of the pond
(on top of the overlapping liner) I have placed rocks. Some of these
are Cotswold sandstone type rocks I believe. I found them all lying
around the garden so I don't know for sure, but my neighbour says they
are. He also says it will kill the fish if you put one of those rocks
in the water. Questions:

1. Is this true?
2. Is it also dangerous to have these toxic rocks around the edge? It
is possible some lime or whatever could filter into the pond through
rain or overflowing.
3. Are the rocks just dangerous to fish? I found several of these
rocks in another old pond around here and there were frogs and newts
living quite happily in it.

Thanks for any answers.

Charlie.