Tanks have unique numbers to them do they not? I suggest doing set prices for people for unlimited fills to people that pre register (they would have to pay for it of course). Perhaps something like 50 bucks or something around that per person or that a pre registered person can buy unlimited fills (for a price that goes down for each tank you buy up to a maximum of x tanks, maybe 5). I understand that it would require a lot of oversite to correctly do that but it could possibly be feasable. This also would only be an option to pre registered people (sort of a hey bonus option for signing up early) so that way you could have time to plan, implement and organize it.