Here is what I found. I have a 2.5 gallon planted and was thinking about rigging up co2 for it. After calculating the cost of a small co2 setup, I decided it was stupid. Think about it. The cost to refill a 9 oz co2 tank at a sports store is like $5, right? I bought a 20lb co2 tank on ebay for about $75 and it costed me less than $15 to get it filled! Bigger co2 tanks have to get hydrotested every 5 years, unlike small paintball co2 tanks. However, even paying $20 every 5 years is still cheaper to just buy a bigger setup. The only reason I would personally pay all the extra money for a small co2 setup is so you can hide it out of sight. Just my 2 cents on the issue.