This is some blurb I found for an Iodide test kit
What I want to know is exactly how much iodide plants and fish need as this makes it seem you need a lot. I'm still having probs keeping plants alive.(Fish are no problem) I've never had a green thumb so plants just don't like me. The CO2 helps but the plants eventualy give up and die. This only happens in a couple of tanks and the water param's are more or less the same throughout.Iodide is necessary for the growth and reproduction of fish and some plants also thrive when iodide is present in the water.