Which glue or epoxy is the best?

[pangking]

Just got some frags in, and thinking of glue them on my LR since they've been knocked down and fell into everywhere all the time. The frags all came with a little rock attached on the buttom.
I don't really want to take my LR out of the tank that might change my aquascaping. Is there any glue or epoxy can do the work underwater? Have anyone tried any method could share here?
Many thanks!