The Expansion Kit is fully compatible with the 12 Site Ebb & Flow Gro System, as well as the 6 Site Gro System. The 6 Site Expansion Kit allows you to increase your yield without a huge investment. The kit comes with six (6) 4 gallon Gro sites and six (6) 3 gallon 360 degree snap-in mesh aeration inserts. These mesh inserts promote lateral root growth and better aeration for the root zone. The snap-in feature keeps your plants from floating in the system. The kit includes 3/4in exclusive Tub Outlet Tee Fitting and tubing to hook up to your system. BPA free.