It's been a while, but I got my tunnels done. I ended up waiting on a batch of black silicone to coat the insides of the caves so the stark white wouldn't stand out so much.
I finished that 2 days ago. I used aquarium silicone, which says it cures completely in 24 hours, but is even safer to wait 48hrs before "filling the aquarium." So, 48hrs to hold under pressure, but that isn't a concern when making caves. Do you think it's safe to put the caves in my tank with my fish now? If I put them up to my nose, I still smell the vinegar smell of the silicone..