Today I was working on a rock structure that was intended to be a hiding place for my Betta's five-gallon tank. It ended up being more of an arch than a tunnel or cave, but that's fine with me.
What I'm curious about is whether or not the structure is safe to put in my Betta's tank. It is composed of hot glue, and those really smooth rocks you can buy at stores. I have it sitting in a sink full of water right now, just to get any loose glue particles of rock pieces off. Here is a photo of the arch-like thing: