I used PUL on the back of bibs--keeps drooly baby shirts dry! It's good to work with but kinda pricey that I wouldn't use it for under the fish tank. I was going to suggest a flannel-backed tablecloth --couple layers together but the vinyl placemat is a good idea, too. That's what I have under the food/water bowls for my kitty but it's on vinyl flooring. I'd be really careful of anything that there is actual wood underneath as any moisture that collects will ruin that. It could collect under the mat you are proposing. Do you have something like a trivet that the bowl/tank could sit up off the tabletop that moisture can evaporate? Look in the dept with flower pots for plastic ones? Just so there is a bit of space between the tank and the wood surface.