MoePaac: Wow, those are really neat ^.^ I wonder if you could tie something like an anubias plant to the PVC cave? The coconut cave, I usually don't care about those too much, but the added java moss just makes it pop somehow and I like it >.> The styrofoam background is just badass, I love it!! Especially since it's much lighter than a real rock... I was wanting to add some larger rocks to a larger tank in the future when I'm able to get one, but I was worried about the weight... I guess with this, I'd be able to make some large fake ones and not have to worry about the weight as much ^.^
I made these two caves. The first one is a rock arch, and the second one is shown in a tank. It's a glass jar that I covered with gravel. I didn't just roll it in gravel, I put the gravel on piece by piece. I also included an up close photo of the glass cave.
I think everyone will agree that aquarium safe silicone is the way to go. However, I have heard some say hot glue is ok to use, others say they wouldn't risk it, some say not to use it.
Personally, I would use hot glue, but I do have silicone on hand and so would use that up first. An idea would be to put it together using a little hot glue (since it dries so quickly) then go around and cover/reinforce with silicone. That may be easiest/best option.