The Two Little Fishies Reducing Tee is a barbed fitting for connecting standard vinyl and soft pvc tubing for canister filters, protein skimmers, calcium reactors, etc. It can also be used for connecting water return lines for sumps and recirculating pumps. Hose slips onto barbed end for secure fit. This 14 mm x 4 mm fits 1/2" ID tubing (and airline/CO2 hose).