The Best Water for Cannabis: RO, Tap, Well, and Distilled Compared
Your water is the base of every feed you mix. If you do not know what is already in it, you are guessing at everything you add on top.
Reverse osmosis: a blank slate
RO strips nearly everything, landing near 0-10 ppm. That gives total control, but it also removes calcium and magnesium, so RO users must add Cal-Mag to almost every feed.
City tap: usable, with caveats
Municipal water typically runs 100-300 ppm and carries chlorine or chloramine. Chlorine gasses off if you leave water out for 24 hours; chloramine does not and needs a carbon filter or a dechlorinating product. High-ppm tap water can push your total feed past target before you add nutrients.
Well water: test it
Well water varies wildly, from 150 to over 800 ppm, and can carry iron, sulfur, and heavy calcium. A cheap TDS meter plus an occasional lab test tells you whether to use it straight, cut it with RO, or skip it.
Softened water: avoid
Water softeners exchange calcium and magnesium for sodium. Sodium accumulates in the root zone and locks out nutrients. Use the untreated hose bib upstream of the softener, or RO.
Distilled and rainwater
Distilled behaves like RO and is fine but expensive at volume. Collected rainwater is naturally soft and slightly acidic, works well, and still needs Cal-Mag and filtering for debris.
Frequently asked questions
- Do I really need a TDS meter?
- Yes. A $15 meter tells you your starting ppm, which is the difference between feeding accurately and feeding blind.