My major concern with using old carpeting (or even new) would be to
ensure what the backing is....some carpets have foam backing that can
become toxic to fish....now of course a good quality weave carpet would
not have these issues but then you need to look at what chemicals have
been used on the carpet itself - so many carpets these days are treated
with stuff to make them stain resistant etc. etc.

Just being cautious here.....but it is a true danger....

If the carpet is going under the liner , how would that be toxic to fish?
Gale :~)