Shine a light at one end of the tank, they will either go towards or away from the light. Clean the area where they aren't. Or, shut off the filter & drop some food in the far back corner of the tank. When they go back to eat, vac the area where the aren't.
You can try using a turkey baster to clean the bottom, or a piece of air line tubing with a loose knot in it to slow the flow so you don't suck up sand. I keep fry in a bare bottom tank, and use the tubing with no knot. Fry get sucked up here & there, I just suck them out of the bucket with the baster & squirt them back into the tank.
Tolak