No offense, but seriously, what is the purpose to this? I mean, what if a fish dies in there? If not a fish, all of that other bacteria and waste will just accumulate.
I mean, I get your idea, but fish are here for our viewing pleasure. If you're going to put tubes under the gravel, why not just put a sheet over the entire thing and run it in your closet? Or what if they love it and all of them never leave the tubes? You could never prove they exist.
Ok, so enough with the criticism, I have an idea. What if you had 2 tanks, let's say 1 on each side of a window and then connected the 2 tanks with the PVC
you were using (or a long clear piece). That way, you could have two completely different aquascapes; like 1 planted and the other all stone and driftwood. This way, the fish could go from 1 tank to the other. There's a bar or restaurant that has this but the tanks are like 30ft apart and clear. I'll try to find a picture and post it.
If you still want to do the undergravel, go for it, it's your setup. I'm just figuring that you will regret it one way or another.