Hydroponic Water Quality: Tap, Filtered, or RO?

Plants don't care where the water came from — they care what's dissolved in it. Here's a no-nonsense guide to picking the right water source for your hydroponic setup.

Tap water — usually fine

If your tap water tests below 300 ppm total dissolved solids and isn't heavily chlorinated, you can use it straight. Let it sit uncovered for 24 hours to off-gas chlorine, or use a cheap inline carbon filter.

When tap water becomes a problem

• Hard water (above 200 ppm calcium/magnesium) throws off your nutrient math.
• Chloramine (instead of chlorine) doesn't off-gas — you need a carbon block.
• Well water often has iron, sulfur, or unpredictable pH swings.

Filtered water

A standard activated-carbon pitcher handles chlorine and most taste/odor issues. It does not remove minerals, so your starting ppm stays the same.

Reverse osmosis (RO)

RO strips almost everything, giving you a clean slate around 0–20 ppm. Worth it if you have very hard water or you're running picky plants like strawberries. Just remember to add a Cal-Mag supplement — pure RO can leave plants deficient.

The simple rule

Test your tap once with a $15 TDS meter. Under 200 ppm? Use it. 200–400 ppm? Filter and adjust. Above 400 ppm? Invest in RO or buy distilled.

Frequently asked questions

Keep reading