thinking about it though, I always see people filling their tanks really quickly from the compressors at my local site... why do they do that? I was always told filling a tank quickly is dangerous because it heats the tank a lot?!
In the past there have been extremely fast fill systems, these had a higher risk of a flash explosion.
Current systems are regulated not only for the maximum pressure, but for the rate of fill.
You can still get a cylinder hot from filling, there is less chance of a flash explosion, but it is not 100% guaranteed, it is safe for cylinders to be filled, however if someone wrongly uses oil on their cylinder and it gets inside then air under pressure, heat plus oil can cause an explosion (you produce the same reaction as a diesel engine)
A cylinder is best filled without it getting hot. Start the fill, pause after a short while if filling from low, and repeat.
If the air gets hot then you get less pressure as it cools, so that is an extra reason to fill slowly.
All cylinders in paintball are tested to pressures that exceed 3000psi or 4500psi so they are safe from failing even if you over pressurise. However the burst disk is designed to fail long before the cylinder.
What this means when you fill is you must fill at the correct fill station for the cylinder - 3000psi or 4500psi.
When using a scuba cylinder then the majority of fill stations are unregulated. This brings back the potential of a high speed fill, therefore open the cylinder slowly and regulate the fill rate yourself.
You could also be using a 3000 or 4500psi (232 or 300bar) scuba cylinder to fill a paintball cylider of either 3000 or 4500psi
4500psi/300bar scuba cylinders are more expensive so less often used for paintball fills.
You could fill a 3000psi paintball cylinder from a 4500 cylinder, but its not recommended.
If a 3000psi regulator is used in the fill rig then it is perfecetly safe for a 3000psi cylinder, without a regulator you can still do it CAREFULLY watching your 3000psi cylinders gauge.
The reason why a 4500 scuba source would be used for 3000psi fills is that the source pressure lasts longer to maintain fills at 3000psi
If you use a 3000 scuba cylinder then your first paintball fill will be very close to 3000psi, the next a fraction less, and less at each fill. If you fill to a lower pressure, e.g. 2500 then the source maintains presure and you get consistent fills.
Also if you fill from empty then you get less fills, if you regularly top up then you use less from the source to maintain consistent pressures.
Check out the UKPSF HPA1 on air safety:
http://www.oaklandsfestival.host56.com/web_documents/air information.pdf
Check out this guide video on filling from scuba cylinders: