CO2 vs Flourish

CO2 is carbon. Carbon is the basic building block from which all life is created. The flourish provides mineral nutrients. CO2 is also a nutrient but of a different sort. Co2 gas gives plants a usable source of carbon. Mineral nutrients & carbon are converted by the plant into actual plant matter. Without a source of carbon, little or no growth can take place.
Low light tanks very rarely benefit from additional co2. The general way it works is the more light you have, the more energy the plant will have at it's disposal. The more energy a plant has to use the faster it can convert mineral nutrients & co2 into plant matter. A deficiency in any of these , mineral nutrients, carbon or light will result in slower plant growth & possibly plant health if it is a severe shortage of any one of those things.

Unless you have lots of plants & lots of light I probably wouldn't be too worried about adding co2. You could always try using Seachem Excel instead of Flourish. It's essentially Flourish but with a liquid source of carbon for the plants to utilise. I've heard of people having trouble with it through over dosing so if you try the Excel just make sure you don't overdose. It can put your fish at risk of oxygen starvation.

From: http://www.seachem.com/support/FAQs/FlourishExcel.html

Q: Is Flourish Excel a replacement for CO2 in a planted aquarium?

A: Yes and no. It provides the same benefit as CO2, i.e. it provides the plants with a source of carbon for growth just as CO2 provides them with carbon. However CO2 by itself will give you quantitatively more growth than Excel by itself, although Excel does provide a substantial amount in comparison. If CO2 is a 10, Excel is a 6-7. Using both together provides additional benefit. One of the advantages of the Excel is no up front equipment costs and complexity of valves, hoses and regulators, etc.

Q: Will an airstone or surface agitation decrease the effectiveness of Excel as it would decrease the effectiveness of CO2 injection?? What effect would an airstone/agitation have on Excel's effective CO2 levels?

A: An air stone or other surface agitation will not impact the effectiveness of Flourish Excel. Flourish Excel is not CO2, it is simply a readily available source of carbon. This is an intermediate source and more readily available than CO2 and will not interfere with the uptake of CO2 as a carbon source.