Polyurethane varnish for tannins?

Epoxy has been used to seal various things in the aquaria like diy styrofoam rock walls like at http://www.csd.net/~cgadd/aqua/art_other_netmax_styro.htm

I havent heard anything about urethane myself.

Epoxy will leave a very shiny unnatural finish though, I'd say the same of varnish. I'm not sure of varnish being aquarium safe. And epoxy is a real pain to work with, dangerous too. It would certainly seal the wood, but you'd need to make sure the wood was dried out completely before applying. If the wood doesn't naturally sink, you'd need to attach slate to anchor it as well.