Start somewhere toward the front you can see there are no shrimp. As you move the vac, keep it close to the gravel. The shrimp will move. If there is a stubborn one, shake the gravel tube a little, it should move. When I gravel vac in my crypt stand, I use a bucket and look for any I sucked up. Usually find a couple. Much of the time they'll swim against the current and back down onto a plant where they can hold on.