This is our favorite and most recommended background foam. Unlike other expanding foams, this one is specifically designed for fish and other animal's safety. It is intended for aquatic applications, so works perfectly in a vivarium. It's black, meaning much more subtle than the other products out there that yellow over time. It's also much more durable, and adheres to glass better.
We recommend spreading a thin layer of silicone over the areas of the tank to be covered, allowing it to cure for 24 hours. After, you can apply this foam in the shape you'd like! Best results when you use 2 or more layers, letting each fully cure for 24 hours (remove skin after 18 hours on each with a knife). Finish by mixing substrate with black silicone and smoothing it all over the background!
This 12oz can will cover 4-5 square feet.