fish aren't that smart and don't have opposable thumbs. i suspect even out of water, there's a good chance they'll severely burn themselves far before they get the hang of the glue gun. i probably wouldn't risk it.
I think he might have the tank dry or it is something like a decoration or something that needs to be put back together. I have tried using hot glue while it is wet. It doesnt get a good bond to whatever you are gluing and it either just doesnt work or is very weak. (not in a fish tank, on an rc boat)