Oxygen Shields are made out a recycled rubber polymer and are designed for years of operation with minimal maintenance. Each Oxygen Shield diffuses air at around 3000 microns or 3 millimeters in diameter. Oxygen Shields come in 5 different sizes and fit up to 70 gallon tanks. The beauty of these manifolds is the low diffusion rate needed to fill the Shield. OS1 is for 10-30 gallon tanks. OS2 is for 20-40 gallon tanks. OS3 is for 40-70 gallon tanks. OS5 is for 5 gallon bucket.