The 75lb tanks can be a bitch to manuever around. I suggest a few 20's if you don't want to go too often. Even the 20's are 45lbs when full, 75's are 213lbs full. I used to have a bunch of 50's (140lbs full), too much work in lugging them around (and I can carry a pair of 120lb dumbells around the gym no prob).
Paintball guns moved away from CO2 to compressed air years ago. Those "paintballers" aren't what they claim.
Local Hydro stores just swap them out, pub supply fills. I prefer the swap, Hydro store already knows what's up, pub supply employees/patrons are the ones to worry about. Either way, to this day, I still make sure that I'm not being followed (only happened once, years ago). It's isn't any cheaper fill VS swap (at least, around here it isn't, Los Angeles). If you want to fill, it's for the tap you have in your "bar".
Tried running natural gas burners. Not really saving there, constant worry about the possibility of fire.
We want "fire", but don't want it in flames until good and ready.
CO2 has been a pain, but we have a "love, hate" relationship.
Another thing, you have to be real careful with it. You have to ensure a good seal with every "connect" and the regulators only last around 5 years before the plastic "flow" gauge degrades (minor cracks) and begins to leak, I've fixed a few, but isn't worth it. One time an entire tank leaked (reg flow ruptured after I hooked up new tanks and left). Tank size and the fact that that room was completely sealed meant that there was enough CO2 in that room to knock me unconscious within 30 seconds, dead in about 20 minutes (which would happen easy after the 30 second KO, not waking from that nap). I "caught it" within 5 seconds of entering the room, still had a mean headache, felt weak and nauseous. Plants were kinda stressed, drooped for a few days, all of the "Mystery" through some nanners, and one HPK grew a full "male flower".
Anyhow, be mindful. Even with controllers in place.