Davy wrote:

I intend to put stones or pebbles in my wildlife pond which won't have any
fish apart from maybe the odd stickleback. My preferred pebbles are
limestone but I have seen a warning against using limestone pebbles
because
they can change the ph. Should I take this warning seriously?

Absolutely not. Limestone in its various forms _will_ move pH towards 8.4,
which is not ideal for some plants or fish, _but_ it will stabilize the pH
in that region, which is much better than having wild swings in the pH.
--
derek