Getting the glue to work under water is a bit tricky but here is what I do.
Gently pat the bottom of the frag dry. Put a thick/tall puddle of glue on the underside where it looks like it will eventually sit on the rock. Test fitting the frag where you want it in the aquarium is helpful and lets you see what part of the frag will touch the rock(and where to initially place the glue).
Take a piece of 1/8-1/4" wooden dowel and build up some of the gel on the end...sort of like a soft serve ice-cream cone. Use the dowel to apply the glue under water. As soon as the gel hits the water it will skin over and should not fall off. Place the glob-o-glue
on the rock scraping it off the dowel. Try and be careful not to spread it around any more than you have to.
Now you have glue on the frag and underwater on the rock. Place the frag on the rock, into the glue, and gently twist. When you twist the frag you will break the skin on the glue and the two liquid centers will mix.
Here is the trick. Once you get them together and in a solid/semi-solid resting place. LEAVE IT ALONE
It may seem like it is not very secure but it will take several minutes for the glue to set underwater. When it does it will be hard and the frag should be secure. To many times I played with my frags underwater and turned the glue into a mass of skin strings
thinking the frag had to be rock solid before the glue had a chance to dry/cure.
After a few minutes(5-10) you can GENTLY push on the frag. If all goes well it will be very solid.
JME
SiF
PS sticking them in a crevice and allowing them to encrust is also a very viable option...when you do, try putting a little gel on the end going into the rock it will help secure it and provide a more mated fit with the rock.