With that level of lighting CO2 would be beneficial, I'd also make sure you get some decent liquid fertiliser aswell, SeaChem products are good and cheap since you need little of it even though the bottle appears expensive. The Nutrafin yeast-based CO2 method is really cheap (eBay UK has them for £17 delivered), and would really help with the plant growth at your lighting level.
Many plants will grow well without a 'proper' substrate providing they get enough food, any plants that prefer food from the roots can be fed using fertiliser tablets that you push into the sand/gravel near the roots.
Carbon is one of those 'dodgy' questions....if you use it then it needs changed regularly otherwise it leaches stuff back into the water. For planted tanks it can also strip out much of the nutrients needed by the plants and it will take up the liquid plant fertiliser aswell. Personally I don't use it and if you want plants then you'd (in my opinion) be better to take it out: the plants will absorb loads of stuff when they grow and this is a better and more natural approach for helping keep the water clean.
What is the plastic tube held down by the ceramic tube?