To make waterproof rock structures you're probably going to need to steer away from the usual Great Stuff foam route of making rocks and instead consider making them from fiberglass mesh and resin [a difficult and time consuming process with a STEEP learning curve], or look at some of the expoxy products sold by Polygem: www.polygem.com/zoo/zoopoxy.php - these materials are also hard to work with and have a steep learning curve as well. If you do some searching on this forum for fake rocks you'll probably come up with some posts by a guy who does some nice work using concrete and metal lath/screen forms. That is also time consuming and difficult, but produces nice results. Concrete is not a good choice for a standard glass aquarium.
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Matt Campbell
25 years herp keeping experience
Full-time zookeeper
Personal collection - 21 snakes (9 genera), 20 lizards (4 genera), 6 chelonians (2 genera)