I used IC Gel all the way down to the end of the tube. You just have to apply constant pressure to the tube to make sure water or air do not get in the tip. Also the cap must be all the way on. If you are careful you can get everything out of the tube.
spend the few dollar's more and get the ic gel. hof carries it, i've used the other's super gue gel ect, for the amount you get and the strenght of the ic it's the best
You can apply under water but you need to apply constant pressure to keep water and air from getting into the tip of the tube. I still glue everything outside of the tank though.