Start by finding out what is already there.
Your water can contribute calcium and magnesium before a bottle is opened. The amounts vary between sources. Your feed and growing medium may contribute more. A name such as “tap water” does not tell you how much of either nutrient you have.
A University of Massachusetts water study found substantial differences between irrigation samples. The useful lesson is to analyse your own source, rather than assume that every tap—or every well—provides the same starting point.
If you are investigating a problem, bring the water analysis, complete feeding recipe, medium and recent observations together. Adding a supplement is a decision to make from that picture, not a reflex triggered by a photograph.
Next step: list every source of calcium and magnesium in the grow before deciding whether a supplement belongs.
Reviewed 8 October 2026