What type of diffuser are you using? I've got a GLA Atomic regulator. They're kinda pricey compared to some others out there like Aquatek, but they're excellet. I highly recommend. They make one that fits on a paintball tank, but you can also get an adapter somewhere else and use it with that in case you plan to upgrade to a larger CO2 tank in the future. I used an Aquatek regulator for a while and it did a decent job, but I'm thrilled with the performance of my GLA regulator.
http://greenleafaquariums.com/co2-regulators.html
http://greenleafaquariums.com/co2-regulators.html