It would swell and probable get a smelly fungus on it, plus i wouldn't be to kean on water soaking it to the timber (mainly softwoods because of the resin's) and finding its way into the aquarium
Finishie depends on what you want it to look like when its finished.
If your after a natural finish try wax, but be careful if you do as you would have to melt it and paint it on and wipe it off strait away, a mix of carnivore and bee's wax is best about 70% to 30% bee's.
You can buy special waxes bout i don't think they give as good a finish.
Alternatively polyurethane or marine varnish are your best bet.
As for wood a ply would be best, preferably something like birch multiply its a good strong ply easy to work with cuts clean and would mould.
You could get a nice modern look with it, if you sand and varnish the edges, or alternatively you could glue wood beads to the edges for a fancy more traditional look.
If you PM me with your ideas and what your after doing, il try my best to help.
