Understood. How do you intend to seal the inside of the tank? The sink is no problem, but a closed tank? I can see you glassing two halves, or a basin and a lid; then bonding them together; then pouring a goodly amount of resin in and tumbling it around to seal the inside seam... but will that really be good enough to be sure no moisture ever gets into the wood?
I'm not trying to poo poo your idea; just trying to understand (and hopefully help you, too) the pitfalls that need to be considered and overcome. Perhaps use a lost foam process and ditch the wood altogether?