I'm upgrading to a slightly bigger tank and want to add a rock arch. Can it been done while the old Rock work is submerged? If so what should I use to glue it?
Yes you can. I used Two Little Fishies brand AquaStik underwater epoxy putty. It comes in 2 colors.
It will make you skimmer overflow for a while so you might want to turn skimmer off for a day or so. Good luck!