You may have a faulty gravel vac. Are you submerging the entire tube down to the airline and pumping it? Is the outlet end below the tube? You have to pump it a few times while fully submerged, mine usually takes 6-10. Get a bucket and set it on the ground in front of the aquarium, submerge the tube and pump while watching the airline you should see the water come through in bits until it runs out the end, once it is running out the end the siphon has started and it will run on it's own. To answer your question, ghost shrimp are good at cleaning floors as well as snails.