Cut a small chunk of PVC pipe that is approximately twice the radius of the mushroom and about 1 to 1.5 inches high (depending on your water flow where the mushroom is going to be placed. The piece of PVC sits around the shroom and reduces the current that might blow it away while still providing enough current above it to keep it happy. As long as the shroom is going to be placed on a rock where the piece of PVC won't slide off, you should be good to go. If it's in a place where it would slide off, then place the shroom on some small rubble rock with the PVC around it, and then move the rubble rock to the permanent rock where you want the mushroom once it attachs to the rubble rock. Keep the rubble rock small enough so that the entire foot of the shroom won't fit on it, that way the foot still has room to attach to your main rock later.