Well, if it's the same stuff I'm thinking of, there isn't a toxicity issue with it. There's folks on the SW side of the hobby that have used it to make real "panels" to reef tanks and embedded pieces of dry coral, etc. to it. I don't recall what the experience was with it over the long term though; as in did it start to break down, etc. But bear in mind that FW and SW do very different things to the same materials.
I would be curious to read what the experience has been wit it on the FW side.
As a suggestion, I think you can accomplish a great deal with just flat rocks, a tube of siicone adhesive, and some imagination. A limiting factor may be finding just the right pump.