Fish Safe Removable Putty?

It's actually a horrible bond. You won't have much trouble breaking it later on. Just bear in mind that bits and hunks of it will adhere to one surface or the other and may take some work to remove completely if that's the goal. Its' "strong suit" is the ability to "adhere" to irregular surfaces, but it's a surface bond that isn't all that durable. Some folks that need more adhesion will actually put super glue on either side of a wad of epoxy and then place that "sandwich" between two irregular surfaces to enhance adhesion.

Epoxies were used extensively on the SW side a long time ago to attach small coral fragments to aquascape-size coral rock. It was replaced a while ago with the far superior performance of super-glue gel formulas. But the epoxies still have maintained something of a following to keep stacked rocks from tumbling. But even there, it didn't work well with the largest of rocks since it doesn't bond all that well. So in the case of much larger rock and stone aquascaping, the practice of drilling the biggest rocks to accept acrylic rods (most use plastic coat hangers) became the ultimate solution. If you think about that for a minute, you may start to envision some incredible, if not surreal, rock formations that could be created.