If you're concerned about sucking something up, you can put a nylon pantyhose over the gravel tube. I sometimes do this with fry tanks, just to be certain.
I would think that pantyhose would reduse the suction to much, but if it works for you. When I clean my fry tank (every day ) I feed them right befor I do it so they are still eating food off the top, but I have to be faster than the food because when the food falls to the bottom, so do the fish. My fry are just to curious to be left alone during cleaning time