i can't speak to the brand you bought, but the one i have (yagote) seems to have a total tolerance of about 2mm. i've seen stuff sucked up in the gap, mainly hemianthus glomeratus leaves after trimming, but not that much is diverted. it is tricky getting the balance between the surface skimming and that which is drawn from the bottom. to avoid air getting sucked in, i adjust it so that the water level in the outer tube is about 1" below the water surface. if you find that the skimmer cup sits too high, draw out some or all of the air under the cup; i use a curved piece of airline tubing attached to a syringe.