You probably don't need co2, for you have low light.The amount of co2 needed depends on the amount of light you have. If light over tank is low and no natural light, there is enough co2 in the water. Basicly co2 has to balanced with the amount of light. A co2 replacement is flourish excel which inadvertly kills algae, most of the time. Some plants are sensitive to it.
You ought to look at the diy light fictures. There are some suggestion there how they set up shop lights over the tank. I beleive I saw 1 at wal-mart for about $20.
Most important nutrient plants need is iron. I learned this from rs79. Perhaps adding iron instead of the spikes would help.
The plants that are perishing need a rich substrate. The hygrophila put out roots on the stems, thus they can get nutrients from the water.
Here is some information concerning the cost of laterite and florite. Laterite greatly reduces the cost of setting up ideal plant growing substrates. For example: It takes at least 4-5 bags of Flourite to setup a standard 55gallon aquarium. At $18-$23 a bag, that's at least $90.00 to shell out. Using the best brand name Laterite (Duplarit) on the other hand, costs $32.00 for a single bag that can be mixed with the bottom 1/3 layer of your pre-existing "pet store" aquarium gravel. Duplarit also comes with proportionally blended nutrients to mix with the laterite.
I have seen laterite at petsmart.
Some people make diy fert balls using osmocote, which can be found at hardware store. More info at this site-
http://aquaria.net/articles/DIY/plants/clay-balls/
The best supstrates are eco complete, onyx gravel, and soilmaster select.
If you can afford to order products look at pet solutions.