With regards to HPA tanks (compressed air or pure Nitrogen) there are a number of points to keep in mind.
All tanks must be periodically retested depending on the material and size. Information for this can be found at this link: CO2 and HPA tank hydrotesting
The fill assembly that the HPA tank uses is unique to HPA tanks only. CO2 tanks use a different method which is specially suited for them only, hence the reason the HPA fill method is different.
All current HPA tank fill assemblies (aka fill nipples) function using a special quick-disconnect male fitting that has a poppet inside to control airflow. These are generally low maintenance parts, however may require infrequent o-ring swapping if a leak occurs.
There are two principal types of fill assemblies. I don't know the actual name for them so I just refer to them as piston-type and seat-type, based on the shape of the internal poppet itself.
Most piston-type poppets use a size 6/70 o-ring to seal, whereas most poppet-types use a 6/90. There may be exceptions to this, but all things equal I've found that anything other than that will be very difficult to work with and you may wish to switch to a more traditional fill assembly if you're unable to repair your current one.
From what I can tell, the piston variety seems to be more commonly used. Smart Parts switched to the seat version in 2005.
In order to access the fill assembly, you will need to unscrew it from the tank using a 7/16" wrench (usually). The poppet will be accessible through the back.
Reinstallation can be tricky, and you may wish to have your local shop do it so they can have it done correctly the first time. It often requires a bit of experience to get correct. When reinstalling the fill assembly you must be sure not to screw it too tightly onto the reg, else the fill poppet will be stuck in the closed position and you won't be able to fill the tank. The seat-type assembles are easier to install since the poppet moves freely; the piston-type assembles stay in place as you install them, so it can be difficult to judge the distance that it has been screwed inward.
I suggest either green (wicking) loctite or teflon tape. Red loctite often doesn't work, and blue definitely doesn't work. Loctite sticks never work regardless of the grade.
Like I said, hand it off to your local place if you don't want to mess with it.
HPA Tank Output:
Tanks always default to a certain output pressure that will be sent to the marker's regulator (or straight into the marker if it doesn't use a regulator). The tank output was once 800-psi since this is roughly the same pressure that CO2 exists at, given reasonable conditions (weather, elevation, etc). In 02-03 most developers started selling "low output" tanks that output at 400-psi instead of the higher pressure of the HP output tanks. The sole reason for the use of LP tanks was so they would be compatible with a certain type of WDP Angel using their WDP mini-reg, which necessitated the sub-800 psi input pressure.
All other markers, without exception, are made to function primarily off HP output (800-psi) tanks. If you are purchasing a new tank then I suggest purchasing an Hp output unless you directly plan on using it with an Angel mini-reg. There's no practical reason whatsoever to use LP input instead of HP, and some LP tanks will cause velocity dropoff due to starving the marker. The fastest of the HPA preset tanks will work okay but there is no reason to use them because the performance will be the same compared to an HP output.