If you want a tank 300 bar that will only fill guns, (only have to have it tested every 5 years instead of 3) London paintball Supplies will help you out.
Helpful guys, Chris or Jon, depends if you have a 3000 psi tank or a 4500psi though.
It isn't cheap but 300 bar tanks are hard to find these days and having you own fill is so much more convenient.
12ltr refills cost about £2.50 a go.