There are a few things you should know as a buyer about paintball propellant sources. The main concepts include hydro date, functionality, type of propellant, and condition. Most tanks, with the exception of chromoly CO2 tanks, have hydro dates. A hydro date is the date that the tank will be safe to use until it needs to be retested. You can get your tank retested at your local airsmith's shop, or some paintball proshops. When buying an air/CO2/nitro tank off of anybody or any site, ALWAYS ask for the hydro date. If a tank is past its hydro date, it is illegal to use until you pay to have it retested. On average, it costs approximately $25 to get a tank retested. NEVER buy a tank that is past its hydro date.

Types of Propellants:

1. CO2 - Most commonly used air source because it is cheap to buy tanks and cheap to fill. New CO2 tanks generally cost approximately $15 for a 9oz, $17 for a 12oz, $19 for a 16oz, and $23 for a 20oz. The more ounces of CO2 a tank can hold, the more shots you will get per fill. One of the disadvantages of using CO2 is that it is actually in super-cold liquid form while in your tank. Only a fraction of the CO2 is in gas form at any given time. When you fire, the gas is drawn into your marker. When you continue to fire at a high rate, the tank gets colder and colder until the liquid does not have enough time to change to a gas, and you will experience damage or a freeze-up. Freeze-ups cause your marker to malfunction for about 15 seconds. It can easily damage most markers, with the exceptions of spyders, piranhas, and tippmanns. Dick's Sporting Goods fills 20oz tanks for about $5. They actually have a price range dependent on how many ounces of CO2 you need. 19oz fills cost about $1 less than 20oz fills because they are in different categories. Not to encourage lieing, but I usually say that my tank has an ounce or two left in it so I don't get charged $1 more.
   
2. HPA/Nitrogen - Very common among tournament players. Both gases are very consistent, causing greater overall accuracy. No matter how fast you shoot, most quality HPA/Nitrogen tanks will maintain a steady input pressure for your marker. HPA/Nitrogen do not cause freeze-ups, so they are considered much cleaner gases. They are always in gas form as far as paintball is concerned. The downside to HPA/Nitrogen is that the cheapest NEW tank you can find costs $63. I am afraid that listing a link is against ebay rules, so you will have to search around. HPA/Nitrogen tanks come in two different pressure settings. 3000psi, the lower of the two, is most common and normally costs about $90 less (cost of tank) than the other. 4500psi, the one commonly found in competition-level players, allows for more shots per fill. A 72 cubic inch (ci) 4500psi system will almost always yield more shots per fill than a 72ci 3000psi system. It usually costs $3 to fill a 3000psi system and $4.50 to fill a 4500 psi system ($1 per 1000psi).

Types of HPA/Nitrogen Tanks

1. Aluminum - Heavier and cheaper. It's much more durable than the other type of tank, but it cannot hold as high a pressure. Usually has a 5-6 year period before the hydro date. Aluminum tanks commonly hold 3000psi.
   
2. Carbon Fiber - Very light, very expensive. It's somewhat vulnerable to scratches and damage, so most people buy tank covers (about $8) to protect their carbon fiber tank. Carbon fiber tanks are the ones that can safely hold 4500psi.

I hope this is helpful to many!