The HPA tanks have a date on them. Based on that date, you can just do some basic math to figure out when the hydro expires. For example, mine says 04/07. Since I have a Carbon Fiber Wrapped tank, it will last 5 years from that date before I have to get it hydro tested again. So I'm good until 4/12. Some tanks have a 3 year hydro on them, such as Fiberglass wrapped tanks. When you buy your tank online or in a store, they should be able to give you this information. Also, in terms of weight, Steelies are the heaviest followed by FiberGlass and Carbon Fiber (lighest).