I agree the loctite gel is good but doing the math I found out it's much more cost effective to use
Ecotech Coral Glue
Loctite $5-6 for 4ml
Coral Glue $22 for 75ml, or $55 for 295ml
It works very well, as long as you give the rock a quick brush first
I tried the
Boston Aquatics stuff, but it doesn't work very well under water