My brother in Calgary has used spray foam twice in his 90 gallon tank. He claims that it dries inert but still manages to break down over time. If you apply it too quickly or thickly, it will likely warp once the water is in the tank. It is also not immune to breakage, scrapage, gouging etc. Which looks a bit unsightly. He always used the "Mono" type foams that come in a can. There may be other types of foam out there that will stand up better in an aquarium.
In my experience as an insulator for a while, I should tell you though that no foam dries completely inert. At least, not when used as it was intended. They ALL off gas over time, and some more than others. There are open cell foams and closed cell foams, sugar based, and poly based. I for one, wouldn't trust any of them in my tank.
Just my 2 cents about the foam thing....back to you Tony!
