Cool and interesting. Just curious if you have some sense of load this structure is going to place on the silicone and/or back glass. I have seen something like this before and it was gorgeous.
Intuitively, the load from the structure is "inward" while the load from the water is pressing outward so maybe they cancel. IDK. Also, if the joint holds while empty, it will certainly hold when water is in there. Is the cleat made of glass? I guess I've read enough about that bond strength (glass/silicone/glass versus glass/silicone/acrylic) to wonder aloud.
Sorry so many questions, but assuming you pull this off I may want to copycat you in some form.
The structure itself only weighs around 21lbs, and should be quite a bit more buoyant in water with how light this rock is.
The rock itself is designed to put stress on knuckles and triangles leading back to the acrylic plate it is mounted on.
The plate that the structure mounts to that is siliconed in place is made out of thick acrylic and is quite large, roughly 12x20 plate, as far as the bond between them I have no doubt it's more than strong enough to hold the plate to the glass without any issue, if it's good enough to hold my tank together the amount I used should never be an issue.