Toadie, your idea is neat and after first reading it, I thought to myself, "Yes, PVC pipe should be fine in standing water." Then I decided to find out if there was any info. out there to back up my initial thought. Here is a PDF file. you can open it with adobe acrobat reader.
http://www.ppfahome.org/pdf/pvcpipewaterspec.pdf
It basically gives PVC pipe specifics and states that yes it is safe to use in potable(drinking) water. I'm assuming you won't have any harsh chemicals in your tank. Your amphibs waste won't become too toxic as long as you clean your tank regularly, which I am sure you will.
I hope this helps. I can't wait to see pics of your a setup.