From everything I've read you want at least 1lb per gallon. I've never seen anything about having too much. My advice would be to start off with 50lbs and see what kind of room you have left. If you're buying it locally I would advise that you go with smaller pieces........that gives you more options of how to stack it. Also, make sure your rock on the bottom is touching the glass of the aquarium and not resting on sand. This will make sure you don't have any shifting and possibly rock slides.