Think of your aquarium as a table's legs. There are four. Those four are: Lighting, CO2, Ferts and Water Changes. If you take away one of those (except for lighting), the the table can still stand up, but, it can easily be destroyed if it becomes unbalanced. So, if you only have your lights, ferts and water changes, your tank would most likely survive, but it can easily be destroyed. (you see what I mean)
If it's a heavily planted tank, then yes, you would need CO2 just for the plants. If there isn't a lot of plants, then you don't necessarily need it. Your fish should be able to give enough CO2 to the plants. If you have a small tank, you can usually use DIY CO2 (for 20G and lower) for tanks bigger than 30G, pressurized CO2 is prolly more effective.
