If you use normal crete and mix then yes it may harm your fish/plants. You
can buy a special mixture to put in the concrete mixture to make a safe
solution preventing the lime leaking out. Take a look at
http://www.seapets.co.uk/product-det...de/440825.html its a uk site
but if you live elsewhere in the world I'm sure you can get a similar item.

Regards

Craig

"gary caunt" wrote in message
...
HI,, i need to put a brick edging around an already established pond.
Can anyone tell me if the mortar will damage the fish/plants?