unclenedsfishfactory.com

PVC pipe
is totally inert
(just plastic)

you can pretty much put any of these in your tank:
plastic
cured epoxy
stone
aquarium driftwood
glass (just watch out for cutting edges)

things to watch out for are:
metals (don't use any)
some species of wood (I had problems with local oak, maybe it was too green and sappy?)
poisoned products (got some cheap chinese decorations once with poison paint on it,
one time a customer used slate from an old roof that had poison adhesive on it)