Is hot glue safe to use on aquarium decorations?

It's better to use silicone or something similar, but you can definitely use hot glue as long as you let it dry. My cichlid-keeping friends hot glue rocks and gravel to their fake plants all the time to keep them weighted down, or glue rocks together to keep them from collapsing if the fish dig. None of them have ever had a problem with the glue in their water. I'd also make sure it's really clean though, without any of the little glue threads that often form when you're using it.

It's better to use aquarium silicone. but I know of many people who have done it without problems. Just glue everything outside of the tank, and make sure it is fully dry before putting it in the water.

I've never used it, but I know someone who has used hot glue to build breeder nets and other things to go inhis aquarium, and he hasn't had any problems with it, so I'm going to say it's safe, as long as you let the glue set before you put it in water.

Keep it far away from your aquarium!

Use aquarium sealant/silicone.