Tap water is any drinking-quality water from a municipal supply or a private well. For growing, its relevant properties are EC, pH, alkalinity (bicarbonate), hardness (calcium and magnesium), sodium and chloride, and the disinfectant used: free chlorine or chloramine. These vary by region, season and source.
In practice
Start with the facts. Municipal suppliers publish annual water-quality reports listing hardness, alkalinity, sodium and disinfectant. Well owners should get a lab test. Then measure your own EC and pH at the tap.
Rough guide: tap EC below about 0.4 mS/cm with moderate alkalinity (40–100 mg/L as CaCO3) is easy to use straight. EC above about 0.8 mS/cm, alkalinity above about 150 mg/L, or sodium above about 50 mg/Lbeing reviewed suggests acidifying, blending with RO water, or switching to RO, especially in hydro and coco.
Hard tap water brings useful calcium and magnesium; account for it before adding Cal-Mag.
Don't confuse with
- RO water: purified, near-zero EC.
- Rainwater: low EC and alkalinity, but can carry roof contaminants.
Myth check
"Letting tap water stand for 24 hours makes it safe" is only partly true: it removes much free chlorine but not chloramine. Check your supplier's report.
EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.Alkalinity is water's capacity to neutralise acid, mainly from dissolved bicarbonate and carbonate, reported in mg/L as CaCO3; it decides how much pH Down you need.Sodium (Na) is not essential for cannabis; it enters through water, softeners and some inputs, and at high levels causes salt stress and crowds out potassium and calcium.Free chlorine is the disinfectant (hypochlorous acid and hypochlorite) added to many tap waters; it dissipates from standing or aerated water within about a day.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.General hardness (GH) is the calcium and magnesium dissolved in water, expressed as mg/L CaCO3 or German degrees (dGH); hard water supplies some Ca and Mg.Chloramine (monochloramine) is a long-lasting water disinfectant made from chlorine and ammonia; unlike chlorine, it does not leave water by standing or aerating.
