I haven't used it personally....yet. I just got all of my equipment to start building my first vivarium today. I'm starting tomorrow with the False bottom and the marine epoxy. I'll probably start some Great Stuff work as well.

This stuff is definitely safe. I know many many people who use it. It's almost a must for backgrounds, etc on the vivarium forum I'm a part of. Like I said I haven't yet used it myself. However, I'll share what I have learned from others so far. First of all, if you plan on using it for a water feature you should go with a type of GS that is water resistant (they don't come waterproof). I have the kitchen and bath type.

When using this stuff try to have an approximate plan as to what you want to make (size, etc). It does expand so you're best starting out with a single layer at a time until you're use to it (I'll probably be a lunatic and just go all in tomorrow knowing myself lol). You want to make sure it is 100% cured before you mess with it. The back of the bottle says to wait a certain amount of time for it to cure. However, that is for it's intended purpose and most people double the curing time as it is typically in a much thicker coat than somebody would use to fill a plumbing gap. If it is not fully cured before you mess with it you can end up with it warping what ever its on, oozing the inards (ew), etc. While none of that is harmful it's disappointing.

From what I understand you can use a template if you want a hole, or if you want a certain shape (such as a hollow "cave"). Let it cure for about half of the time and then remove the "template" and allow it to finish curing.

Once its cured you can sculpt it with a regular seraded kitchen knife. Shape it however you'd like. Then you want to use silicone (GEII is what has been recommended by others). You want the entire thing coated in teh silicone to make it waterproof. people use different methods to decorate the pieces. For instance, the waterfall I posted above has sand and little pebbles. I will be using gravel and river rocks for my GS pond, and I will be adding beak moss or coconut fiber (haven't decided yet) to my GS backgrounds. To do this, as soon as you apply the silicone stick the grave/sand/moss/etc into the silicone. When the silicone dries the materials will be glued in place. For the rocky hide I posted the person used JUST silicone to create that. Instead of using clear they used black and white silicone. They mixed the two colors in different amounts to get highlights, shadows, etc and "painted" the piece with the silicone.

When I start my vivarium I'm going to be taking pictures of the entire process. I'll share how the great stuff part goes! Once this vivarium is complete I'm going to do over the tank that the anoles and gray tree frogs are in with a FB/water feature/GS background etc. I'll also take pictures of that whole process and will post all of that on here as it is for my garter (already bought a nice piece of slate for basking).

Feel free to PM me if you want me to put you in touch with more experienced people on the matter