Water quality in hydroponics (chlorine, hardness, RO water)

Water isn't just water

Hydroponics is 90% water and 10% minerals — yet most beginners never look at what kind of water they pour into the tower. And water sets the foundation: if it already contains chlorine, limescale or too much salt, you start in the red before adding a single drop of fertiliser. The good news is that water is one of the easiest things to check and fix.

In this article we look at the three things that affect home growing in Croatia the most: chlorine and chloramine, water hardness (limescale), and when and why to consider RO (reverse osmosis) water.

Starting EC: measure your "empty" water first

Before anything else, measure your water's EC before fertiliser. That's your "starting EC" and it tells you how many minerals are already in there.

Why does it matter? If your water already reads EC 0.7 and lettuce needs 1.0 total, you have only 0.3 of "room" for the actual fertiliser — and that "room" is full of calcium and magnesium that may not be in the ideal ratio. More on EC in the EC and pH guide.

Chlorine and chloramine

Municipal water is disinfected with chlorine or chloramine to make it safe to drink — but those compounds can harm beneficial microbes and irritate tender roots.

• Chlorine is volatile. If you leave the water in an open container for 24–48 hours, or aerate it with an air pump, the chlorine mostly evaporates.
• Chloramine (chlorine bound to ammonia) is more persistent. It does not disappear by standing or boiling. It's removed with an activated carbon filter (carbon/KDF) or a trace of vitamin C (ascorbic acid).

For a home tower, tap chlorine concentrations are usually low and plants tolerate them, but if you notice stalled growth or slimy roots, leaving the water overnight or a cheap carbon filter often solves it. You can check the disinfection type with your local water utility.

A practical trick: fill a bucket of water the day before you need it and let it stand uncovered. The chlorine evaporates, the water reaches room temperature (so it doesn't cold-shock the roots), and fine debris settles to the bottom. One small habit, three benefits.

Water hardness (limescale)

Hardness is the amount of dissolved calcium and magnesium, usually expressed as mg/L CaCO₃ or in German degrees (°dH). Much of Croatia — especially the karst hinterland and coast (Kvarner, Dalmatia, Lika) — has hard to very hard water thanks to limestone bedrock.

Why hard water is a challenge in hydroponics

• It raises the starting EC, leaving less room for fertiliser.
• It pushes pH upward and makes it harder to hold the 5.5–6.5 range (the water is "buffered" by carbonates).
• Excess calcium/magnesium can upset the ratio with other nutrients and deposit limescale in the system.

Light to moderate hardness is actually helpful — it brings free calcium and magnesium. The problem only appears with hard and very hard water.

RO water: when it pays off

Reverse osmosis (RO) forces water through a membrane and removes over 90–95% of dissolved minerals, leaving near-pure water with EC close to zero. That gives you a "blank slate" — you decide exactly which nutrients enter and in what ratio.

Consider RO water if:

• your water's starting EC exceeds 0.7–0.8 mS/cm;
• hardness is above ~250–300 mg/L CaCO₃;
• you're fighting a pH that constantly creeps up;
• you grow sensitive crops (strawberries, some herbs) that prefer clean base water.

Caution: pure RO water has no calcium or magnesium at all. If you switch fully to RO, your fertiliser must supply them — quality complete formulations (like the dojdi mixes) already contain them, but some "lighter" fertilisers assume Ca/Mg comes from the tap.

A practical compromise: blending

You don't have to go "all or nothing". A very popular and reliable approach is to blend tap and RO water (e.g. 50:50) until you reach a starting EC of roughly 0.2–0.4 mS/cm. That keeps some free calcium and magnesium while trimming the excess.

Rainwater is also an excellent, soft base water — provided you collect it from a clean surface and pre-filter coarse debris.

Water temperature — the often-forgotten factor

Warm water (above ~24 °C) holds less oxygen and favours algae and root disease. Aim for 18–22 °C in the reservoir. In summer keep the tower out of direct sun, and add an air pump if needed. More on system cleanliness in Cleaning and algae.

A quick action plan for your water

1. Measure starting EC and pH of your tap water.
2. Find out the hardness (utility data or a cheap test) and the disinfection type (chlorine/chloramine).
3. If EC < 0.5 and hardness is moderate → use the water as is, optionally leaving it overnight for chlorine.
4. If EC > 0.7 or the water is very hard → blend with RO or switch to RO plus a complete fertiliser.
5. Keep the temperature at 18–22 °C.

Key takeaways

• Always measure the starting EC before fertiliser — it's the foundation of the whole solution.
• Chlorine evaporates after 24–48 h of standing; chloramine needs a carbon filter.
• Much of Croatia has hard water; moderate hardness is a benefit, very hard water is a challenge for pH and nutrient ratio.
• RO or blended water pays off when starting EC is > 0.7–0.8 mS/cm or the water is very hard — but then the fertiliser must carry calcium and magnesium.
• Keep the temperature at 18–22 °C and hold pH at 5.5–6.5 (see the EC and pH guide).