I have a 150 gallon tank that I am in the process of turning into an aquatic turtle tank. I plan to have a large plxiglass platform in the back center but the problem I am having is how to build a waterfall mountain. I have a fluval FX5 that I will be using for filtration. I plan to make a large plexiglass column in the corner to run my return into and I wated to build a mountain around it so the water can overflow and run down the mountain. I was wondering if anyone had any ideas about what to build the mountain out of. I was playing with the idea of an aquarium safe cement (if that exists) but if I end up wanting to use the tank for something else later I dont want anything permenentin there. I need some ideas bigtime.