Before you buy one, make sure you have a place lined up to refill it. Most of the welding shops, hydroponics stores, etc. around here only do trade-outs and only carried 20# tanks. I did find a fire extinguisher outfit that would do refills, so I ordered a 10# aluminum cylinder from one of the sponsors for around $100 with shipping included. Consensus of opinion appears to be that CO2 tanks need to stand vertically to prevent liquid CO2 from entering the regulator, so if you go for a 20# tank, make sure you've got room for it.
Well, at least my algae is pearling...