First, I want to say I'm sorry. I saw this yesterday and tried posting, but my internet access at the University timed out and it didn't go through.
Just because you opt for low-light doesn't mean you shouldn't not use CO2. In fact, it's great to use CO2 in low-light situation. I personally don't right now, but often when I first setup a low-light tank, I will inject DIY CO2. I don't know the why's about it, but I find that the tank runs much better and I rarely have the major algae problems that people get. After about 6-9 months, then I take away the CO2 but I still get only very mild algae. You can totally opt to keep the CO2 after that, and I'm sure I wouldn't have any algae if I did this, but I am extremely lazy. You can measure your levels accurately with a drop checker and I highly recommend that you purchase one. I don't have one, but it's one of those things I wish I had had. Takes the guess work out of determining your CO2 level.
For lighting, wpg and that sort of stuff isn't all that important anymore. Obviously I am able to grow things quite well without much light at all and SuperColey1 even grew R. macrandra (a supposedly high light plant in .9WPG). Also, save yourself some money and opt for either regular T8s or better yet, regular T5s, not HO. The important thing isn't wattage necessarily, but even distribution of light and good spread. Unfortunately many HO lighting and CF fixtures cram the bulbs into such a small space that you don't get very even coverage. With a larger distance between bulbs, there is more overlap and you get better light coverage at least that is what I understand. T8s come into play (I know right, dinosaur lighting) because the twin T8s actually have some decent spread between the tubes and cover more of the width of the tank. I am currently using just simple twin T5s, not as much spread, but better penetration as my tank is already quite low in light. The light fixture should be rougly the length of the tank. Unfortunately, lights aren't really built with this kind of spread in mind, so DIY might be an option to get this result. It also helps if the lighting is suspended, but again, I do not do this because I am lazy. LEDs are also a new option, and several members here are using them in their planted tanks, but right now, I am way too lazy to invest the time or effort in creating my own LED fixture and I don't think you absolutely need them to grow plants well. There are some good lighting options out there, but don't go crazy. Even if you decide on higher light levels, I don't think you'll need to go above 2wpg. You'll be able to grow everything really without much issue, especially if you inject CO2. If you go low and inject CO2, almost the same story, except the grow will be slower. Low light and no CO2, you're more limited with what you can grow, because there are plants that do much better with CO2 injection, but you'll be able to grow the hardies (crypts, swords, mosses, ferns, etc).
There are other factors to consider like ferts and substrates, but really lighting and CO2 are issues that should be addressed first. Once you determine those factors, the others are easy.
Hope this helps.
llj
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