Thanks Fibber.....I applied it directly to the glass. It is the same stuff as "Great Stuff" You use it to seal gaps on windows, electric boxes...etc.
The total cure time is 24 hours. After about 30 minutes, the outer shell if you will, begins to harden. You have some time to play with it and I wish I would have done more manipulation of the foam to make crevices and cliffs. Lesson learned for the next tank.
It is really neat stuff. The snails, stars and hermits climb all over it.
Good luck with your 120. I think you will like the foam back if you decide to go that route.
Todd