Any kind of CA glue, CYANOACRYLATE ADHESIVE, is fine in your tank. Krazy glue is CA. Gel is the best - I use it all the time to glue frags. Dry the part of the frag you want to glue down, put a small bit of glue on it (if you use too much it takes a while to dry), push the glue against the surface you want the frag on and hold it for about 30 seconds. This can be done in the tank.
Many people use and recommed using CA glue to hold frags. It is also mentioned in much of the literature.
The bond of CA glues can be fragile in your tank. If you bump it, it will come apart. It is best used to hold frags in place until they grow onto and over the glue bond.
Some stores sell 'reef safe' glues, but they are just CA glue as well.
RR