I bought myself the entry level Red Sea reactor kit. What it comes down to is that it is way too much money for what you get, but at the same time it's easy, comes with everything you need. Anyway, my tank is 20 gallons (about 75 litres?) and it keeps my co2 levels at 20 - 15. Although they say it should last 2 to 4 weeks, I find I need to do it every week to keep from levels sinking below where I want them.
Luckily their "refills" are nothing more than yeast and sugar, so it is still cheap.
Your tank is quite a bit bigger and to make it more you would surely need two of them. A better idea might be to look at the DYI options online, you can use a standard plastic pop bottle as the reactor, and a small powerhead, or even a canister filter output, to disperse the gas.
Obviously a co2 tank with all the goodies is the superior option, but it ain't cheap. Biological reactors work just fine for us poorer folks.
