If it's a small tank, ~$60 will get you a decent paintball CO2 system. $150 is what it will take to get a pressurized system. Might seem like a lot, but they really are worth it. When you consider that a quality 4' T5HO system will set you back $100-$200 easily, and the Doublebright LEDs that people keep getting are more than that, suddenly it doesn't seem so bad. But because people overvalue light, they see a $150 lighting system as a worthy investment, and a good CO2 system as needlessly expensive. Not only that, but in a 50g tank, a CO2 system will break even with Excel dosing in two years, less for larger tanks (and really, once you break out of low light, you should be dosing some form of carbon).