Join Date: Jan 2008
For a 55 gallon aquarium, I would not recommend that you go with paintball CO2, and instead, get a larger (i.e. 5 or 10 pound) CO2 cylinder instead.
This is mainly because you will end up running through paintball cylinders relatively quickly. The cost for CO2 refills generally does not differ very much for larger cylinders, so in the end, it is more economical to get a larger cylinder.
For regulators, I usually recommend people assemble them from parts off eBay, since it can be cheaper (especially if you bargain hunt). For more information regarding that route, take a look at my Primer to Pressurized CO2 (linked in my signature below).
For diffusers, are you specifically looking for an in-aquarium one? ADA, Do Aqua, GLA all sell nice diffusers, but I find them on the expensive side. I have been using some cheap no-name one that I got overseas, and it has been working fine.
If you have a canister filter, you could also consider inline and/or (DIY) CO2 reactors, so that less equipment is inside your aquarium.