It depends on how much pressure you want in your dive tank...
If you are htinking you can use your fathers garage tool compressor at 250psi then you will be sadly dissapointed.
Usually around 3000psi minimum upto 4500 psi depending in the type of tank.
If you are filling a dive tank you should only use 'clean' air from a dive shop just in case someone accidently uses the dive tank again for diving...
It's not worth the risk really. Get it filled properly at a dive shop.
manike