The light is probably a 15w T8, which gives you 1wpg, at this lighting level no CO2 is necassary, and only little fertilisation is needed.
If you upgrade to a T6 tube (from aqua essentials) this gives out 40% more light, which will give you 1.4wpg, which IMO would be the best thing to do. It will fit straight into the existing T8 ballast, Again CO2 is not necassary although it will still help, there is the option of DIY:
http
/www.fishforever.co.uk/carbondioxide.html
http
/www.fishforums.net/index.php?showtopic=96754
but this can cause algae if it fluctuates which it probably will, there is also the option of a pressurized system, these are quite dear but you have optimum control of the amount of CO2 you inject. Ready made kits are available, or there is also a fire extinghuisher (see 2 guides in my sig) you can set this up for around £80 with a 2kg FE.
http
/www.fishforums.net/index.php?showtopic=105266
The 2 best fertilisers on the market are seachem flourish, and tropica plant nutrtion+ (aqua essentials) TPN+ is easier to work out a measurement for daily dosing which is better for plants, 1ml per 20l
You may need to supplement your substrate with root tabs which need replacing every 3-4months.
Is there anything else you need to know?