The shells will provide a source of calcium, probably not needed, and a
source of carbonates, which is very desirable. The carbonates will dissolve
when the water starts to go acid, stabilizing the pH. As long as the pH is
high, the sea shells will not dissolve, but if the pH starts to decrease,
then having the buffer is good. One problem with putting them in the pond,
if you are putting in enough to cover the bottom of the pond is the amount
of debris that will be trapped, and due to poor water circulation the
development of some nasty bacteria.
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html

"not very clever" news at snorkerz.com wrote in message
...
I have a new pond (approx 5000 litres) with a pump/filter system with a
capacity up to 12000 litres.

I intend keeping Goldfish in it once everything is balanced out.

Would seashells (cockle) be okay in the base? I have read somewhere that
they are made of calcium which might be a problem - or would the filter
sort that out?

Thanks for your help:-)