Drylok is fish safe but I wouldn't use it to waterproof a plywood tank. I've seen a couple of failed drylok plywood builds. There were a few sections near the top of my paludarium where I used drylok to seal the wood and they started molding and rotting after a few months (and that wasn't even submerged, just high humidity).
Apparently it works better if you put a layer of concrete backer board on top of the plywood and then apply the Drylok over that, but at that point it's probably no longer cheaper. For waterproofing plywood tanks I would stick with the tried and tested epoxy or a liquid rubber product.
very nice job. It looks like the tank sits directly on those 4x1s. I would put plywood between the stand. That makes for a better weight distribution. Maybe even add some foam like a camping mat or styrofoam