In article , Diane Epps
writes
I made it formal with steep sides a
metre deep and have never had a problem. The herons come and look but can't
get at the fish.

You did put a bit of a ramp in it somewhere, didn't you, so that any
animals that climb in it can climb out again?

--
Jane Ransom in Lancaster.
I won't respond to private emails that are on topic for urg
but if you need to email me for any other reason, put ransoms
at jandg dot demon dot co dot uk where you see