Yeah they epoxy is strong. The thing is - with all the rocks bonded - I moved the based rock back by an inch on both the left side and right side.
So at that time, perhaps the bonding broke and hence the collapse.
Also if you epoxy rocks, it works great. but if you are trying to epoxy a small frag with minimum rock onto a base rock in the tank water itself, it is very difficult.
you will need a tweezer/pincer/ cotton bud to push the epoxy to the bonding surface.