Water in Hydroponics: Complete Guide to the Optimal Choice

In hydroponic cultivation, water is not just a vehicle for fertilizers — it’s the foundation upon which the plant’s nutrient balance is built. Choosing the right type of water in hydroponics— whether reverse osmosis or tap — is one of the most important decisions, especially for those seeking professional, consistent results. In this article, we’ll explore the pros and cons of both options and how to evaluate which is best for your system.

Why Water Quality Is Critical for Cultivation

Water is the central element of every hydroponic system. It not only transports nutrients, but also directly influences root health, nutrient availability, and solution stability. In soilless systems, where the substrate is absent or inert, even small variations in water quality can have a significant impact on crop performance. For this reason, it is essential to constantly monitor the main parameters — especially electrical conductivity (EC), which indicates the concentration of dissolved salts.

Tap Water or Reverse Osmosis? Choosing the Right water in hydroponics

In hydroponics, the two most common water sources are the municipal tap supply and reverse osmosis (RO). The latter, produced through an advanced filtration system, has an almost zero EC and allows total control over nutrient formulation. It is ideal for automated systems or sensitive crops like baby leaf and microgreens.

Main advantages of RO water:

Complete control over dissolved salts
No interference from sodium, chlorine, or calcium
Maximum stability in dosing and pH management

However, it also comes with some drawbacks:

Higher initial system cost
Significant water waste (2–3 liters discarded per liter of purified water)
Regular filter maintenance required

Tap water, on the other hand, can be a valid alternative — but only if its EC falls within an acceptable range. In such cases, it can be used directly or mixed with RO water to reduce salinity and improve quality.

When and How to Use Tap Water

In many areas, tap water is clean enough for hydroponic cultivation, especially for less sensitive species. However, to determine suitability, it is essential to measure the EC — the electrical conductivity — which reflects the total amount of dissolved salts. This allows growers to decide whether to use it as-is, mix it with RO water, or fully filter it.

To test tap water easily and accurately, you can use the EC probe integrated in the NIDO ONE V2 system. Simply remove the probe from the holder, immerse it in a container with tap water, open the NIDO app, and select the “Refresh” function. Within about 10 seconds, the measured value will appear in the app, helping you determine the best strategy for your crops.

Regular EC monitoring, especially with seasonal changes, helps prevent imbalances. If the value is slightly above the ideal range, mixing tap and RO water is a great solution — lowering total EC, retaining beneficial minerals, and optimizing control while reducing waste.

Below is a reference table that helps you evaluate your for water quality based on EC values:

EC (mS/cm)	Rating	Recommended Use	Suitable Crops
< 0.2	Excellent	Ideal for all crops	Strawberry, lettuce, basil, baby leaf
0.2 – 0.5	Good	Suitable with calibrated fertilization	Tomato, pepper, cucumber
0.5 – 0.8	Acceptable	Only for hardy crops or mixed water use	Eggplant, cabbage, spinach
> 0.8	Critical	Avoid; use RO or mixed water	Fodder crops or highly resistant ones

When Water in hydroponics Should Be Exclusively Reverse Osmosis

There are cases where using only RO water is strongly recommended, if not essential. These include technical, professional setups, closed-loop systems (such as NFT or DWC), or cultivation of crops highly sensitive to saline imbalances. In these situations, even minor impurities in tap water can disrupt the nutrient solution, affecting fertilizer absorption or pH stability.

Using only RO water allows you to:

Maintain full control over nutrient solution quality
Avoid salt buildup in closed systems
Reduce wear on pumps and hydraulic components
Improve yield and uniformity

→ As a general rule, the more technical and demanding the crop, the more it is worth starting from pure water.

Conclusion

Water quality is one of the most underestimated yet essential aspects of soilless cultivation. Regularly measuring the EC of your source water helps avoid errors in nutrient solution formulation and tailor fertigation strategies to your plants’ real needs.

Choosing between tap, RO, or mixed hydroponic water is not a one-time decision, but an ongoing process that depends on available water quality, crop sensitivity, and the level of control desired.

A smart, informed approach to water management is the first true step to building a hydroponic system that is efficient, sustainable, and high-yielding.