Water quality for hydroponics
Hard water, soft water, well water, city water. What's in your tap affects everything you add after. When to filter and when to work with what you've got.
Your water is the foundation of your nutrient solution. Everything you add (nutrients, pH adjusters, supplements) sits on top of whatever your tap water already contains. If your water has 200 parts per million of dissolved minerals before you add anything, those minerals are part of your nutrient budget whether you planned for them or not. If it has chloramine that kills beneficial root bacteria, that's a problem you need to solve before worrying about NPK ratios.
Know what you're starting with
Get a water quality report. If you're on municipal water, your utility publishes one annually (usually searchable on their website). The numbers that matter for hydroponics:
Total dissolved solids (TDS) or EC. This is the overall mineral load. TDS below 50 parts per million (EC below 0.1 mS/cm) is very soft. TDS 50-150 parts per million is moderate. Above 200 parts per million, your water contributes meaningful nutrient content that you need to account for.
Calcium and magnesium (hardness). Hard water is high in dissolved calcium and magnesium from limestone aquifers. Calcium above 70 parts per million may require using a "hard water" nutrient formula to avoid excess calcium in the solution.
Alkalinity and pH. Alkalinity (bicarbonate content) determines how strongly your water resists pH change. High-alkalinity water (above 150 parts per million as CaCO3) buffers against pH adjustment, making it harder to bring pH down and keep it stable.
Chlorine and chloramine. Municipal water is disinfected with one or both. Chlorine offgasses if you let water sit uncovered for 24 hours or aerate it vigorously. Chloramine (chlorine bonded to ammonia) does not offgas and requires chemical treatment (sodium thiosulfate, sold as aquarium water conditioner) or activated carbon filtration to remove. Chloramine damages root cells and kills beneficial microbes.
Sodium. Elevated sodium (above 50 parts per million) is problematic. Plants can't use sodium, and it accumulates in the root zone, raising osmotic stress. Common in water softened with a sodium-exchange water softener (don't use softened water for hydroponics) and in some well water sources.
City water
Most municipal water in the US has a pH of 7.0-8.5, TDS of 50-400 parts per million, and is treated with chlorine or chloramine. For hydroponics:
If TDS is under 150 parts per million and chloramine is absent: you can use it directly after letting it sit to offgas chlorine. Add nutrients on top of the existing mineral content.
If TDS is under 150 parts per million but chloramine is present: use an aquarium dechlorinator (sodium thiosulfate based) before use.
If TDS is 150-300 parts per million: use a "hard water" nutrient formula that accounts for the calcium and magnesium already in your water. Monitor EC carefully since you're starting with a higher baseline.
If TDS is above 300 parts per million: consider RO filtration or blending with RO water to bring the starting TDS down.
Well water
Well water varies wildly by region. It can be soft and acidic (granite aquifers) or extremely hard and alkaline (limestone aquifers). It may contain iron, manganese, sulfur, or other minerals at levels that interfere with hydroponics.
Get a thorough water test before using well water. Lab testing (about $30-80 for a hydroponic-relevant panel) gives you the full picture. At minimum, test pH, TDS, hardness, iron, and sodium. If iron is above 0.3 parts per million, it can precipitate as rust in the reservoir and clog drip systems.
RO water
Reverse osmosis filtration strips nearly everything from the water: minerals, chlorine, chloramine, heavy metals. The output is typically 0-10 parts per million TDS. This gives you complete control over the nutrient composition.
Advantages: Start from a blank slate. No unknown minerals, no interference from chloramine, no hardness to work around. Consistent results regardless of tap water variability.
Disadvantages: RO units waste water (typical recovery is 25-50%, meaning 50-75% of the input water goes down the drain). You need to add everything, including calcium and magnesium that tap water would have provided for free. The unit itself costs $50-150 for a basic countertop model, and filters need replacement every 6-12 months ($20-40 per set).
For growers with hard tap water (above 200 parts per million TDS) or those who want absolute consistency, RO is worth the investment. For growers with moderate, consistent tap water, it's optional.
Blending
A practical middle ground: mix RO water with tap water to reach a target starting TDS. If your tap is 300 parts per million and you want to start at 100 parts per million, mix 1 part tap to 2 parts RO. This reduces the unknown mineral contribution while retaining some of the calcium and magnesium you'd otherwise need to add back.
The EC to PPM converter helps you track your water's starting mineral load and calculate how much nutrient room you have before reaching your target EC.
Seasonal variation in municipal water
Tap water composition isn't constant. Municipal suppliers adjust treatment seasonally, and source water changes with rainfall and snowmelt. Spring runoff dilutes minerals (lower TDS), while late summer draws from deeper reserves (higher TDS). Chloramine dosing may increase after heavy rain to compensate for increased organic matter in the source water.
This means the nutrient solution you mixed perfectly in January may behave differently in July with the same tap water and the same nutrient concentrate. If you notice unexplained pH drift, EC discrepancies, or plant behavior changes mid-season, retest your tap water before assuming the problem is in your nutrient formulation.
Growers who run the same system year-round sometimes see problems in spring or fall that disappear in summer. This often tracks with municipal water changes that aren't visible without testing.
Testing your water
TDS meter (EC/PPM pen): The fastest test. Dip the pen in your tap water and read the total dissolved solids. Under $15 for a reliable unit. This gives you the overall mineral load but doesn't tell you which specific minerals are present.
pH meter: Essential for hydro regardless of water quality concerns. Your tap water pH is the starting point before nutrient addition. Most municipal water runs pH 7.0-8.5. pH pens cost $10-30.
Test strips or liquid kits for hardness: GH and KH test kits (sold for aquariums) tell you how much calcium/magnesium (GH) and carbonate (KH) your water contains. Inexpensive and quick. If GH is above 150 parts per million and KH is above 120 parts per million, your water is hard enough to warrant using a hard-water nutrient formula or blending with RO.
Full lab analysis: If you're on well water or your municipal report doesn't include the detail you need, send a sample to a water testing lab. A hydroponic-relevant panel (pH, TDS, calcium, magnesium, sodium, iron, chloride, alkalinity) costs $30-80 and gives you definitive numbers to formulate against. This is a one-time investment that answers the "what am I working with" question permanently (unless you change water sources).
When filtering is and isn't worth it
If your tap water is under 150 parts per million TDS, has chlorine (not chloramine) as the disinfectant, and you're growing leafy greens or herbs, you probably don't need any filtration. Let the water sit overnight to offgas chlorine, add nutrients, and grow.
If your tap water is above 250 parts per million TDS, contains chloramine, has high sodium (above 50 parts per million), or has iron content that leaves orange stains on your sink, filtration improves your results significantly. A basic 3-stage RO unit ($50-100) mounted under a kitchen sink provides clean water on demand and pays for itself in more consistent crops and less troubleshooting.
The middle range (150-250 parts per million TDS) is where judgment comes in. Many growers in this range succeed with tap water by using hard-water nutrient formulas and monitoring EC carefully. Others prefer the control that comes from starting at zero with RO. Try tap water first. If you hit persistent problems (nutrient lockout, unexplained deficiencies, pH that won't stay adjusted), RO is the solution.
The EC to PPM converter helps you track your water's starting mineral load and calculate how much nutrient room you have before reaching your target EC.