Optical grade magnifying lens with 4in viewing portal is perfect for aquarium photography with focal range approximately 3 to 8 inches. Held in place by strong rare earth magnets for use on tanks up to 5/8 (16mm) tank wall thickness. Easily moves around the tank for viewing from any location. Great at checking for any early signs of disease or pests on your reef coral or fish!Deep See is Safe for both glass and acrylic!