The safest in my opinion would be 100% silicone. You have to make absolute certain it does not have a mold inhibitor. This additive more than likely will make your animals very sick and may kill them. Years ago a friend used it in a salt water setup and it did kill his fish. Another thing to consider. Silicone will not stick to some plastics, rubber. I've learned that the hard way. You will have to ditch the suction cup. Heres the kicker, you will have to completely drain, than completely break down your whole setup. Before applying silicone to decor, glass you have to make sure all surfaces are completely dry and clean. You can use alcohol to clean nonporus surfaces as the last step before applying silicone. You need to find a way to keep decor in place and the position you prefer, while the silicone sets and that may be a bit tricky. Good luck with your issue.