Next place damp paper towels on top and then add the rest of your gravel, about another 2 inches. This will provide iron through the root systems of the plants. The liquid forms can be a bit of a pain as some have a tendency to cloud the water. I have recently been put on to a form that does not. In either case, shopping around and experimenting with some of the forms will help you to find a suitable iron solution.
Carbon dioxide is a tissue building chemical.
During plant respiration, a process also known as photosynthesis, the plant is able to take carbon dioxide from its environment and in the presence of light convert it to a usable carbon tissue and release the oxygen back into the environment. Oxygen This is of little interest to the layman but can provide some insight to the goings on in the plant. This bleaching is called Chlorosis. It is important that the cells stay a nice green color or whatever special color that each plant may be so that the photosynthetic process is not stopped. In some areas your water may or may not contain everything plants need.