This discussion comes up periodically. Remember, plants are largely made of carbon, but they don't consume their own carbon in order to grow, nor do they consume each other for carbon. Fish are largely carbon, but plants don't capture fish and consume them. It isn't just carbon that the plants need, but bioavailable carbon, which is carbon dioxide largely. When plants die and decay, they give off carbon dioxide, and that is used by other plants. So, unless you find an unusual molecule, like glutaraldehyde apparently is, you are stuck with CO2 or something that gives off CO2 as it breaks down, while not being toxic at the same time.