Gotcha! Thanks again!If it retracts to 2", you would need at least a 2" pipe. Trick would be to not have the pipe all of the way down the sponge. Leave a gap from the "base" of the sponge to the rock, so it makes the nem retract and it should be able to crawl up the pipe with no issues.
If no pipe that big, you could even take an old plastic cup, cut the bottom out, sand it smooth where cut and use that to block flow and light. Same idea, but may be something more readily available. Tecik is to have whatever at an angle to make the nem want to move to flow and light.