I see that you were/are worried about co2 injection. If you aren't wanting to spend alot of money on the tanks and pressure regulating system you could always go with DIY. I would reccommend using 2-4 2litre bottles (or juice bottles or whatever you want to use). All you do is cut a small hole in the cap of the bottle, with just enough room to fit some plastic tubing through the hole (cut the tube at a slant, and pull through the other end with pliars so it's nice and airtight.). You want the tube/hose sticking about an inch or two into the bottle. Then you can seal around the hose hole with silicone or anything else. Once you got the bottle setup, fill the bottle about 3/4 the way with warm (not hot, not cold) water. To this you add approx 1 cup of sugar, and 1-2 tsp of yeast. Then you take the other end of the tube that is outside the bottle and hook it up to an airstone/diffuser of some sort and put it into the tank, and there you have your DIY co2. Other methods on getting it diffused into the water may include running that hose/tube line into the inlet of the canister filter or somewhere else where it could diffuse into the water through the filter. This co2 mixture should last anwhere from 1-4 weeks, at which time you will have to empty out the bottle and make another batch. A good way to figure out how long you have left of the batch is to see approximatly how many bps (bubbles per second) the bottle is producing. This can be really easy or more difficult depending on how you sent the tube into the tank. If you just ran the line to the tank or used an airstone then you can just watch and see how often it makes a bubble, usually 1-3 bps from the bottle is good, any less and it's time to make another batch.
Sorry if this is too wordy or confusing. If you have more questions about DIY co2 feel free to ask, or check out the DIY forum I believe there are plenty of threads on that, if not a sticky.