I'm building out a 90 gallon reef tank with a deep sandbed. The tank is 36" deep and 24" square. I plan on keeping a yellow-headed jawfish and likely a goby/shrimp pair. They'll be digging up the sand so I want to anchor my aquascape to a PVC skeleton that will be buried beneath the sand. I don't want them to be able to shift the aquascape around. So what should I do, fill the pipes with sand or just leave them open and let them fill as they may? Should they be water-tight and empty and epoxied to the bottom of the tank? Anybody have some related experience they can share? The bed will vary in depth from 5 to 8 inches.