What size is your tank? What kind of lights do you have?
If you have a low amount of lights, then you can do without CO2. You could look into Seachem Excel which is a source of carbon, but not as good as CO2.
If you have higher levels of lighting, CO2 will start to become necessary.
J, it will depend on your plants. If you have low light plants, they most probably won't need CO2 or additional ferts. Low light = less photosynthesis. Thus, they wouldn't be able to use the extra CO2 or nutrients anyway.
Some people have problems with algae if they leave the light on for 10-12 hours. Some plant people recommend that much time, since they say most of those plants are tropical in origin so in nature would get that much light. I think it's a matter of how you see your plants doing, and if you notice algae. It's hard at first to find the right balance.
Like I first put my lights on for 12 hours and got tons of algae. So I turned it back to 8 hours. The algae went away, but the plants weren't growing as much. Now I've found the right balance of fertilizers, light, and Excel (I use that instead of CO2) and don't have any algae! It makes me very happy to see my healthy, balanced, green tanks It took quite a few months, though, and a lot of trial and error.