The Aquasana Optimh2o Reverse Osmosis Under Sink Water Filter System comes with a 4.4-gallon tank that has a 3.2-gallon capacity and a 5-year warranty. This compatible replacement for the AQ-RO-3.55 OB system features a butyl diaphragm and polypropylene liner that prevent the growth of germs and impurities. It has a sturdy metal shell with a high-grade butyl diaphragm, and is NSF 61 and 58 listed, IAMPO approved, and CE Mark approved. Maximum working pressure is 100PSI and it is pre-charged at 7 PSI. This tank is only shipped to the lower 48 states, so please ask before purchasing if you reside in P.Rico, Hawaii, Alaska, Guam, or US V.Islands.