Paintball tanks come in many sizes, brands, and may have different features. These tanks are split up between Co2 and compressed air(or HPA). Co2 is the same gas you will find powering Kegerators and soda fountains, and is at most playing temperatures, liquid and cold. Compressed air(HPA), depending on the quality of the filter set up where you fill, compressed breathing air like for scuba diving. You can not use a Co2 tank for compressed air, and vice versa. 

Co2 tanks can not be filled with HPA, and HPA tanks should not be used for Co2.

 

Co2 is the less expensive option, but does come with its faults. One of the major benefits of compressed air over Co2 is consistency. Co2 pressure fluctuates with temperature rather than volume, which is why you will not find a Co2 tank with a gauge, and why all compressed air tanks have one. Output pressure also stays more consistent as liquid Co2 will come out roughly at 850 psi, but can expand to a gas at roughly 1,200 psi. The consistent pressure from compressed air will give better accuracy grouping as more shots fall within a smaller feet per second range. Compressed air also leads to less service as it is less harsh on the orings. 

Co2 tank sizes are measured in ounces, while compressed air tanks get a bit more convoluted with two measurements: cubic inches(ci) and pounds per square inch(psi). Ounces for co2 is fairly straight forward, the more ounces the larger volume and the more shots. With compressed air you have the volume, the cubic inch of the tank, and for all intents and purposes the density being the pounds per square inch. Compressed air tanks come in 3000psi and 4500psi, with 4500psi tanks holding one and a half that of their 3000psi counterparts for the same ci. 

There are also differences from compressed air tank to compressed air tank, and regulator to regulator. The most noticeable difference between the tanks themselves is the material of their construction. They are split between aluminum and fiber wrapped/carbon fiber. Aluminum tanks are the cheaper variant and are solely available in 3000 psi, with all modern fiber wrapped tanks coming in 4500psi. Fiber wrapped tanks are not only lighter, but have a higher capacity than aluminum tanks their same size. There are also tank variations in fiber wrapped tanks that can make them lighter and smaller for a given capacity. 

Regulators will also come with varying levels from adjustability. From being preset from factory to allowing adjustment in both pressure output and fit on the gun. While most guns will operate fine on the standard 800psi out put from the regulator, some require or will preform more optimally with a lower output. Lowering the output pressure also allows you to shoot deeper into the tank, meaning that instead of shooting down to 800psi before encountering drop off you can shoot down to say 400psi meaning that you get to use that extra 400psi. Some tanks also let you adjust how the tank lines up on the gun, so you can ensure that the gauge is facing up, and the fill nipple down both making it easier to fill/ view the remaining air and keeping the larger bits on the reg out of the way of your wrist. 

 

 The material that the threaded portion of the tank, called the tank bonnet, is also important. Many tank regulators have an aluminum tank bonnet. Aluminum on the aluminum asa of the gun is more likely to bind if you attempt to install the tank crooked, and more likely to be damaged if dropped compared to other materials. This is why many regs, especially in the higher end market, are coming with either stainless, brass, or even titanium bonnets. These harder materials lead to a longer life for the the tank and the gun asa.