I should have thought of that before I did mine. The foam should last forever I would think, since they use it on docks, but I don't know about the concrete. I've had mine for a couple months and it hasn't changed.
I used a resin from home depot I don't remember the brand name. I didn't think I would ever use it again so I threw it out plus I almost used it all. It was a lot of work! If I had to do it again the way I made mine I would build it in the tank not out. The reason being is mine is not flat in the tank . I angled in the corner so I could hide the heater, hoses and overflow box. If ya have anymore question just ask and there are pics of it on here.