probably anything larger than 2" pipe. I think that you would probably want 3" pvc. The eel will be probably small to start out, but if you are going with a Deep Sand Bed, 3" pipe and elbows should be easily buried. that and if you were to go with 3" pvc, you can put it in your rock to form cave networks for your fish to hide in and stabilize your aquascaping.
Thats my guess on what you would want to get.
-prk543