Get the biggest tank you could fit under your tank. The bigger the bottle, the less you'll have to drive with it in your vehicle to refill it. If you can afford it, buy another bottle and use it as back up when your other tank run out. For my 55 gallons planted tank, I have a 20 pounds bottle and a 3.5 pounds as a back up. Leave the bottle under your tank especially if you have kids. Get them new and find a refill place in town that would be willing to fill it your you on the spot. The beer supply shop next to AquaTek will refill your you. They don't fill with as much high pressure, but they also charge less. Get aluminum b/c they look shiny and pretty; and are less prone to rust. Those old metal bottle might have lubricant inside to prevent rusting, you don't want that to end up inside your tank.