Aside from freak accidents, or very poorly made resin ornaments, I very much doubt resin ornaments are going to harm fish in any way. Re driftwood, the tannin is often not that dark, you can always soak it first in a bucket of water into which the tannins will leach, and waterlog the wood so it sink in your aquarium immediately, Tannins won't hang around forever, a few weeks and nothing's left. Not even that if you soak it in the aforementioned bucket first. Soaking times vary depending on the size and thickness of the wood.