When I first started adding C02 to my 75 gallon I used one 2 liter bottle. It worked fine and my plants grew fantastic because any C02 will help. I found it is a good way to start. Once you figure things out (going hight tech takes time and patience) you can add to it. Pressurized is the only way to go, and that does not mean that you have to run at the max. I run my system at the lowest I can to maintain healthy plants and a stable environment. It is a lot cheaper and easier to maintain. Lots of C02 and light means adding proper ferts which in turn means VERY vigorous plant growth and lots of pruning. It is very nice to look at but with a very high maintenance factor involved.