Every nutrient mix, every pH adjustment, every feed starts with water. And most growers pour from the tap without knowing what's in it. That works — until it doesn't. Mystery calcium issues, persistent pH drift, unexplained leaf burn, or nutrient lockout that won't resolve even after a flush — these often trace back to water quality.
This guide covers what's in your water, what matters, what doesn't, and when you need to treat or filter it.
What's in your tap water
Municipal tap water contains dissolved minerals (calcium, magnesium, sodium, chloride, and others) plus disinfectants (chlorine or chloramine) added during treatment. The exact composition varies by city, season, and source.
The numbers that matter
| Measurement | What it tells you | Good range for cannabis |
|---|---|---|
| PPM / TDS (total dissolved solids) | Total mineral content | Below 200 PPM is good; below 100 is ideal |
| EC (electrical conductivity) | Same as PPM but in mS/cm | Below 0.4 EC |
| pH | Acidity / alkalinity | 6.5–7.5 from the tap (you'll adjust before feeding) |
| Hardness (GH) | Calcium + magnesium concentration | Soft to moderately hard (50–150 PPM) |
| Chlorine | Disinfectant | Any level (easy to remove) |
| Chloramine | Persistent disinfectant | Any level (harder to remove — see below) |
| Sodium | Salt content | Below 50 PPM |
How to find your water data
- Your city's annual water quality report — every US municipality publishes one (search "[your city] water quality report" or "CCR"). It lists PPM, hardness, pH, and disinfectant type
- A TDS meter — $10–15 on Amazon. Stick it in a glass of tap water. Gives you PPM instantly. Every grower should own one
- A full water test — Ward Labs ($30) tests for individual minerals, heavy metals, and more. Worth doing once if you're on well water or have persistent nutrient issues
Chlorine vs. chloramine
These are the two disinfectants used in municipal water. Which one you have determines how you treat your water.
Chlorine
What it is: chlorine gas dissolved in water. The "pool smell."
Effect on plants: chlorine at municipal levels (0.5–2 PPM) is mildly harmful to beneficial soil microbes but doesn't damage plant tissue directly. If you're growing in living soil or with beneficial microbes, you should remove it. In sterile coco or hydro, it's mostly irrelevant.
How to remove it: let your water sit in an open container for 24 hours. Chlorine is a gas — it evaporates. An air stone (aquarium bubbler) speeds this to 1–2 hours. That's it. No filter needed.
Chloramine
What it is: chlorine bonded to ammonia (NH₂Cl). More stable than chlorine, doesn't off-gas, and persists longer in the distribution system. That's why cities switched to it — it's more effective at preventing bacterial growth in pipes.
Effect on plants: similar to chlorine, but the ammonia component can add nitrogen to your solution in small amounts. More importantly, chloramine kills beneficial microbes in living soil and doesn't evaporate on its own.
How to remove it:
- Activated carbon filter — a basic carbon filter (like a garden hose filter or under-sink unit) removes chloramine effectively. Flow rate matters — slow is better for complete removal
- Campden tablets (sodium metabisulfite) — one tablet treats 20 gallons. Dissolves chloramine bond instantly. $5 for 50 tablets. This is the cheapest and most practical solution for most growers
- RO system — removes chloramine along with everything else
- Vitamin C (ascorbic acid) — neutralizes chloramine. 1g per 100 gallons. More common in aquarium use but works
Letting water sit does NOT remove chloramine. This is the most common misconception. Chloramine is stable — it doesn't off-gas like free chlorine. If your city uses chloramine (check your CCR), you need an active removal method.
Hard water vs. soft water
Water hardness measures calcium and magnesium content. It directly affects your feeding because hard water already contains significant Ca and Mg before you add nutrients.
The hardness scale
| PPM (as CaCO₃) | Classification | Cannabis implications |
|---|---|---|
| 0–60 | Soft | Add cal-mag with every feed |
| 60–120 | Moderately hard | May need supplemental cal-mag in coco; usually fine in soil |
| 120–180 | Hard | Reduce or eliminate cal-mag supplementation; watch for buildup |
| 180–300 | Very hard | Consider RO or blending; excess Ca/Mg can compete with K uptake |
| 300+ | Extremely hard | RO system recommended; buildup will cause problems |
Why hardness matters
In coco: coco coir has a natural affinity for calcium and magnesium (it holds them on its cation exchange sites). In soft water, you need to add cal-mag because coco steals what little Ca/Mg your water provides. In hard water (180+ PPM), the excess calcium can saturate the coco's exchange sites, compete with potassium uptake, and cause potassium deficiency symptoms even when K is present in the feed.
In soil: soil buffers hardness better than coco. Moderate hardness (60–150 PPM) is actually beneficial — the water provides baseline calcium and magnesium for free. Very hard water (250+ PPM) causes mineral buildup over time, visible as white crusty deposits on the soil surface and pot edges.
In hydro: hardness matters most. The reservoir is the root zone. Hard water raises baseline EC before you add a single milliliter of nutrients, leaving less room in your target EC range for actual food. Most serious hydro growers use RO water.
PPM: the starting line
Your tap water PPM tells you how much "stuff" is already dissolved before you add nutrients. This matters because your nutrient line assumes a specific starting point.
Below 100 PPM: excellent starting water. You can use it directly. Your nutrient dosing matches the manufacturer's recommendations closely.
100–200 PPM: acceptable. The minerals in your water contribute to the total EC, which means you may need to reduce nutrient dosing by 10–20% compared to label recommendations.
200–300 PPM: marginal. You're starting with significant mineral load. If you're having persistent nutrient issues (lockout, buildup, unexplained deficiencies), water quality is a likely contributor. Consider an RO system.
Above 300 PPM: problematic for most setups. An RO system or blending RO with tap (50/50) will produce noticeably better results.
When you need an RO system
An RO (reverse osmosis) system removes 95–99% of dissolved minerals, giving you near-zero PPM water. It's a blank canvas — you add back exactly what the plant needs and nothing else.
You need RO if:
- Your tap water is above 300 PPM
- You're running hydro/DWC (precise nutrient control matters)
- You have persistent nutrient issues you can't diagnose
- Your tap water has high sodium (above 50 PPM) — sodium is toxic to cannabis and can't be removed by carbon filtration
- You're growing in coco with very hard water (frequent lockout)
You don't need RO if:
- Your tap water is below 150 PPM
- You're growing in soil (soil buffers mineral variation)
- Your current grows are healthy with tap water
- You're on a budget and the water is workable
RO system basics
A basic under-sink RO unit ($100–200) produces 50–100 gallons per day. For a home grow, this is more than enough. A 4-stage system (sediment filter → carbon block → RO membrane → carbon post-filter) handles everything.
The catch: RO water has zero calcium and magnesium. You must add cal-mag back (1–2 mL/L) before feeding, or you'll get immediate deficiency. Many nutrient lines now offer "hard water" and "soft water" formulas — use the soft water version with RO.
Waste water: RO systems produce waste water (typically 3–4 gallons of waste per 1 gallon of pure water). Use the waste water for non- cannabis plants, cleaning, or other household uses.
Well water
If you're on a well, your water quality depends entirely on your local geology. Wells can be excellent (clean, mineral-balanced) or terrible (high iron, sulfur, heavy metals, extreme hardness).
Get a full test
A Ward Labs test ($30) or equivalent tells you exactly what's in your well water. Test for:
- PPM/TDS
- pH
- Hardness (Ca and Mg separately)
- Iron (high iron causes nutrient lockout and stains)
- Sulfur (hydrogen sulfide — the rotten egg smell)
- Sodium
- Heavy metals (lead, arsenic — not for the plant, for you)
Common well water issues
High iron (above 0.3 PPM): iron precipitates out of solution when exposed to air, forming rust-colored deposits. It can clog drip emitters, stain equipment, and compete with manganese uptake. An iron filter or sediment filter helps.
High sulfur: the rotten egg smell (hydrogen sulfide). At low levels it's harmless to plants. At high levels it can inhibit root function. Aeration (bubbling air through the water) removes dissolved H₂S.
Extreme hardness (400+ PPM): common in limestone regions. An RO system is the most practical solution. Alternatively, blend well water 50/50 with RO or rainwater.
Rainwater and collected water
Rainwater is naturally soft (near-zero PPM) and slightly acidic (pH 5.5–6.0 from dissolved CO₂). It's excellent for cannabis — many outdoor growers use it exclusively.
Collection concerns:
- Don't collect from a roof with chemical treatments (zinc strips, algae inhibitors, treated wood)
- First-flush diverters reduce contamination from roof debris
- Store in a dark, sealed container to prevent algae growth
- Use within a week, or add an aquarium air stone to keep it oxygenated
- Add cal-mag (rainwater has zero Ca/Mg)
The practical water setup
For most home growers
- Get a TDS meter ($10) and test your tap water
- If PPM is below 200, you're good. Use tap water, let it sit 24 hours if your city uses chlorine, or use a Campden tablet for chloramine
- Add cal-mag if growing in coco (regardless of water hardness — start at 1 mL/L and adjust based on plant response)
- pH after adding nutrients, not before
- Done. Don't overthink it
For growers with water problems
- Test your water fully (Ward Labs or equivalent)
- If PPM is above 300, sodium is above 50, or hardness is above 250: get an RO system and add cal-mag back
- If PPM is 200–300: try blending 50/50 with RO or distilled water from the store (temporary solution while you decide on RO)
Water quality isn't glamorous, but fixing it often resolves "mystery" nutrient problems that no amount of product swapping can fix. For pH management after you've sorted your water, see our pH lockout guide.