Another water test, and it still won't sink. Still thinking about how to salvage this thing, but I would definitely not recommend anyone use this process for creating an aquarium background/ornament.
Great stuff is way too buoyant, it won't submerge. Structure weighs about 17lbs at this point, not sure how much weight I can safely add.
Another issue I am having is that the concrete is cracking and chipping off when I manhandle it...I hope it will survive in the aquarium.
SO...if anyone is still following my madness, here's what I did next. I tried using acetone to dissolve the great foam, and that didn't do anything. Then I grabbed a chisel and chopped into the top of the cave, the overhang part, and started gutting out more foam. I then filled in the cavity with yet more concrete, and patched up all the cracks and chipped areas.
Now another week to cure again. This weekend I start on the stand.