In message , Broadback
writes

Perhaps not strictly legal, I don't know. But if you have a natural
pond near you collect a bucket of water then tip it into yours. chances
are you will soon have some water snails, and maybe other things. ;-)

That's the conventional advice for getting a pond ecology going, and
it's perfectly legal, though if the donor pond is on private land it'd
be polite to ask permission.

--
Sue ]