If you do decide to seal it epoxy is probably safest for yor fish and the most durable, especially out of direct sun. It's a little glossy though. Awesome idea and build
It doesnt look as good as it does in the pics, but i just put it in and put some water and pulled the sides. The problem is that when i pulled the sides the weight of the water didnt allow it to move much because of the friction inder it