When cyanoacrylate hardens it is completely non-toxic to your tank, and will not breakdown into harmful chemicals in that environment. It has been used for years in the salt water industry for frag propagation and is the go to material of choice for many salt water hobbiests. I would suggest using it over hot glue for any projects that your working on. If you need more working time, you might want to consider finding a marine safe 5 minute epoxy that would give you the working time you might need. Just my $0.02 on this topic.