Rust colored spots on the oldest leafs

Pretty hard to overwater when your growing in water lol. Check the undersides of the leaves with a magnifying glass. If you have mites then mix a little dawnmsoap in water and spray the undersides and the tops of the leaves. Repeat every 3 days as necessary until all have been eliminated. You should only need to add cal/mg at a very small amount if you need to at all. To much cal/mg can cause problems as well. Very high in nitrogen. See how the plant does without it and add some if needed. When growing this way, plants recover very rapidly, so it is best to go with less cuz less is always best.... more only when needed.