Hot glue is actually more then just the crafting glue you buy at the hobby store. It is divided into High temp and low temp melts and you have to have the correct gun for the type of glue sticks. I believe there are also dual temp guns. Thee is a very wide range of glue strengths. There are grades of stick glue that if you applied to porous rocks, such as a lot of the dry rock and tried to pull it apart, the rock would break before the joint would. I have thought about this many times but I always build with wet live rock. As for using on pieces under water, that is probably a no go as no matter how high the temp, the glue will skin over almost instantly on contact with the cool water. Fragging, may also be a possible use. I have personally done repairs to some of the acrylic equipment that use white PVC welding and it worked extremely well. I think the biggest possibility is creating a reef using dry coral rock. Using a high strength stick, the glue will get pressed into the holes in the rock, locking the pieces together and within a few minutes the bond is strong and may be dang near permanent so make sure you have it positioned fairly quickly. I would bet you could make some very intricate reefs using these glues with absolutely no danger of it collapsing even after years in the tank. They are thermoplastics and are fish safe.
