Cyanoacrylic Gel, no other additives, you can find it at your hobby shop, woodworking shops and even the dollar store under the loctite name in single use tubes. This is the best adhesive to use for gluing frags into a wet tank. If you are working outside the tank, you can use that 2 part epoxy putty you cut a piece off a mash up like playdough, but that stuff is very difficult to work with underwater properly.