Planted tank help - Nutrafin CO2 Natural Plant System (Hagen) ??

I'm trying to develop a planted tank in my 55-gal community tank, but my plants don't seem to be flourishing like I want. Right now I have 4 clumps of microsword (still alive but not spreading) for foreground plants, hornwort (I tried anchoring clumps in the gravel, but it rotted, so now it's just floating around the tank, making a mess by fragmenting everywhere), 2 bunches of red ludwigia (one rotted to nothing and the other seems to be heading that way), 2 banana plants (the leaves are turning black and breaking), and one java fern (seems to holding it's own, but 2 leaves are black, not green). I add liquid plant fertilizer after each weekly water change, but it doesn't seem to be helping.

INFO: My tank size is a 55 gal. I have one-bulb hood with a 40-watt 4-foot fluorescent light bulb. My substrate is just small (pea-sized) gravel. I do not have CO2. I'm planning on mixing in some type of plant substrate like Floramax for Planted Aquariums (Question: how much for my size aquarium ? www.thatfishplace.com sells a 40-lb bag for $28). My water source is well-water, which is run through a water softener because of extreme hardness (iron) in our well.

My main question however is about CO2. First, should I have it ? Second, if yes, why are they so expensive ? The CO2 systems I see online and in catalogues seem to be very expensive. However, I saw one online (again at www.thatfishplace.com) by Hagen that seems very inexpensive and I was wondering what other people's opinions on it are. The product is Nutrafin CO2 Natural Plant System (Hagen) and can be seen at http://www.thatpetplace.com/MainPro/...71+0966&PgNo=1

Any help would be appreciated for this planted-tank newbie. I'm so inspired by people's planted aquarium tank pictures on different websites. I'd love to get my tank to even half that !

Thanks - Shawn