I've also wanted to build my own 3d background for quite some time now. Been watching some videos and browsing the threads in the diy section, plenty of good info there.
From what i understand, you generally want to have 2 or 3 sheets of styrofoam, bond them with silicone then use various carving tools such as files and drywall scrapers to achieve the desired look. Drylok and then whatever colorants you want are added, finishing touches are done with fine brushes etc. Am not sure about curing time nor if a sealer is needed. The background is then siliconed to the back of the tank.