Why not just use paintball co2? Have you checked out jagged fury's thread? Paintball tanks are very easily obtainable, most for less than $20' and my local dicks sporting goods only charges $3.99 for a refill. Just a thout. Works well on tanks less than say 30 gallons. But if your setting up a 90 gallon or something either use more, or use a more expensive larger tank like the ones your talking about. But my pressurized co2 only cost me around $40. Good luck!
"Simplicity is the ultimate sophistication." -- Steve Jobs