Algae is caused by a variety of factors. The most common culprits seem to be unstable CO2 levels, CO2 levels that are not appropriate to your lighting levels, lack of good tank circulation, and small ammonia spikes, with the unstable CO2 playing a more indirect role. It isn't low CO2, it's
unstable CO2, which explains why
some non CO2 tanks can be algae-free too. The ammonia spikes are quite small, not at the level toxic to fish. Notice I didn't say excess nutrients. That theory, along with certain types of algae being caused by high phosporus and iron, seems to have been put to bed. At least with regard to planted tanks. It made sense a while back, but with people pumping copious amounts of Nitrogen, phosphorus, Potassium, and other nutrients into their tanks and experiencing no algae if the CO2 levels were stable, people had to think of another reason. I apologize, as this is extremely general. Recently, obtaining
stable CO2 levels (whether they are low or high) has been gaining more importance, out-weighing light levels in some circumstances. It's quite neat. I've been doing some reading.
I have used the Nutrafin CO2 system with great success. I throw away the activator and yeast packets and make my own recipe using the following.
1 cup of sugar
1/2 tsp of yeast (I usually buy just a jar of yeast, it is usually better quality than the packets)
I don't need to use bicarbonate as I have a high kH in my tapwater, but others use it to prevent the yeast from bubbling into the tubing which produces a kind of icky white stuff. I will often hook two canisters to one difusor and alternate canisters to get more stable CO2.
You can use CO2 in high-light tanks or in low-light tanks. For low-light tanks, high levels of CO2 are not as crucial, you won't need 50ppm, but the CO2 does allow for faster growth.
Hope this helps a bit.
llj
/www.thekrib.com/Plants/CO2/co2-narten.html
We're both right, so this could be a long night. 
