Hi, I don't know if you found a sollution for your problem yet.
I used a product from Home Depot on a wood tank I built. It is like an epoxy you mix 2 parts in equal amounts and pour and it dries to a clear plastic like solid in like 12 hours. It is for table tops and is 100 waterproof, like when you see a bar or table at a resurant with dollars or other items embeded in it. Also it will be liquid for a bit and you can move the tank around getting it into all the cracks.
It is in the paint area, and it like $20 for 32oz, I think. That should be enough for up to like 1/2" on a tank that size so you may not want to use it all. I think it was called Epoxy resin.
I used it on the back and bottom of my tank and it so far has been working great.
Hope this helps. You can see my tank pics on the forum here.