I would recommend rubbing a thn layer on the glass before applying the GS. Let the silicone cure, then apply the GS foam. GS wasn't intended for bonding to glass so it can separate without the silicone. Lay the tank with the background facing up. Spray a bit of foam at a time until you get the feel for it. If you want to layer the foam up, let it cure for a day and then add more or it takes FOREVER to cure or it will fall over becoming frustrating. The spray foam is fun to work with.

Let the foam cure. The foam expands a lot and then shrinks a bit. That's why I silicone first. When it shrinks, it stays attached to the glass.

Once the foam has completely cured( about 36-48 hrs), carve it to your liking and sand it down to eliminate the tiny cracks and crevicesformed by spraying it in. The foam dries smooth making silicone a bit tough to spread. If you rough up the smooth spots with sand paper, the silicone holds better and is easier to apply.

Apply the silicone in small sections. I recommend mentally sectioning off the tank into thirds. Silicone, coco or peat. Silicone then coco or peat. Until everything is covered. If you can, use brown or black silicone. This way if you miss a spot, no big deal, it's brown.

SUPER IMPORTANT

WEAR GLOVES AND SKIN PROTECTION.
great stuff is not great on skin. It does NOT come off skin except for peeling off while taking skin with it.

Good luck, take pictures and let us know how you're doing.