Problems with fake foam rock wall

[Willlis]

So i used that black pond spray expanding foam to create a fake rock wall for the back of my two tanks i'm building. Then I covered it with black silicone and sand. Looked pretty good, nice granite like rock texture and color. Added water after letting it cure for over a week and the whole thing detached from the back and floated to the top after about an hour. Any ideas on how to fix this? Anyone else do something similar that did work?

 

[daedae]

Most people use the insulating foam as a background for semi-aquatic setups, like in vivs and such. They don't hold up too great while completely submerged. Thr foam is porous and extremely light, its gonna float

 

[browbli]

Layer of foam, layer of mortar, layer of fiberglass resin. Quality every time.

 

[asfouts]

glue something heavy to it to make it sink maybe? Like a rock that fits with the background. I have no experience with false backgrounds though haha. So my advice may not be best

 

[evut]

Some people have made backgrounds by applying the foam to a polystyrene sheet, which can be attached to glass with silicone.

 

[grius]

Foam mortar or cement/sand bleend is the best, and some type of epoxy to make it secure. You can carve of some of the foam on the back when the rock is finished, that may help the background to sink easier. And a thick layer of cement will also help with that.