Hi guys, thanks for the kind words.
Marc, they're actually vermetid snails, so they are permanently attached. For some reason they prefer the pipe to the rock ... perhaps the CBB keeps them at bay on the rock. One thing that's kind-of-uncool about them is that they send out these little spiderweb-like nets for catching particulates and it can get a little unsightly if any detritus gets stirred up and caught in the webs. But they do make the white PVC look a little more "reef-ified" than otherwise, so I let them be on the outputs and standpipes. Live-and-let-live, biodiversity-now!! .. and all that, ya know