PVC Caves. Risk of anoxia?

Well first of all I am wondering how long a cave has to be (that has no intentional current going through it) before I have to worry about the water not having any O2.

Anyways here is my idea, and I'm sure this idea has been done before because its fairly intuitive. Basically I have some 2" white PVC piping that I am going to coat in GE 1 silicone and then roll it all around in sand, gravel, and anything else I think may look good. I am guessing this will take up a lot of silicone and that I may have trouble keeping the sand substrate one clean of algae once its all done.

So I was wondering if anyone has tried doing this with PVC and what their results were. So far my only concern is that the inside of the PVC will show up all bright which makes it easier to see if its clean or if I have fry magically inside, however it might also make the fish think that its not really that dark in there and not want to hide inside of them.

I have some undecorated PVC in with a pair of convicts right now and they use them much more than I thought they would. However right now its really an eyesore.

Anyone have any tips?