Wow! I'm in the process of making my very first styrofoam project! However, it's for my gecko, so it's in a dry set up. Hmm... I'm not sure how you would go about making one for an aquarium. Obviously, make sure everything you use is nontoxic. What are you coating the styrofoam with? Whatever it is would probably then need to be sealed with something "aquarium safe". You should do a google search, you'll probably be able to find the various products people use.
Sorry I wasn't much help, I just got excited because I'm in process of completing my first project and it was a lot of fun.
