I have found a bit of info on DIY foam backdrops. It sounds like it would be a great idea for the look I'm going for.

Since I will have to do this after the tank is up and running I think I'll build a model of the tank out of scrap wood and layer it with plastic or shrinkwrap then foam in some dry LR then cure and rinse before inserting the backdrop.

Thats the plan anyway. Does anyone have any experience with this sort of thing? And opinions or advise? are there non-toxic adhesives that I could use in a tank to set the pieces in place?