I top it up after every shooting session now.

its not a big problem as long as you don't go down to nearly empty.

If you have not bought the charging adaptor with the rifle, can i suggest you pop down to a local hydraulics coupling firm !
basically u need 5/8 BSP straight thread adaptors to fit the bottle, plus a down step to fit hose on the pump and washers to suit ! - it set me back 5 quid from my locate supplier !!

Trust me- Bob and all the converted, you will love the stealth !

Regards
Nigel