420 Science Rez Block is produced to hep keep your bong clean. Available in 3 sizes.You may change the water everyday but that isn’t enough to stop gunk building up inside your bong or water pipe. Tar stains not only look unsightly but also taste foul. Why ruin your smoke? Use Rez Block in the water to really cut down on build-up and maintain a sweet-smelling and tasting bong. Rez Block makes the residue and particles float in the water instead of adhering to the inside walls and is then easily poured away!Rez Block is available in the following sizes:Rex Block Mini: 3 ml.Rez Block: 15 ml.Rez Block XL: 30 ml.