Reverse Osmosis Hydroponics
How to Use RO Water for Hydroponics: Setup, Benefits & Best Practices
RO water for hydroponics works by forcing tap water through a semi-permeable membrane that strips out chlorine, heavy metals, excess salts, and nitrates, leaving you with a near-pure base to build your nutrient solution on. Because the water starts clean and predictable, your plants take up nutrients more consistently and you spend less time chasing pH and EC swings. Most serious hydroponic growers switch to RO water once they realize that tap water’s variable mineral content is the hidden cause of many nutrient problems.
Why Use RO Water for Hydroponics?
Tap water is not a blank slate. Depending on where you live, your water supply can carry chlorine, chloramine, calcium, magnesium, sodium, nitrates, heavy metals, and residual pesticides. When you mix nutrients into water that already contains unknown amounts of these minerals, the math stops working. You think you are feeding your plants at EC 1.8, but you are actually feeding them at EC 2.3 because the tap water contributed 0.5 mS/cm of its own dissolved solids before you added a single drop of nutrient.
RO water removes that variable. A standard RO membrane strips 95-99% of dissolved solids, dropping your source water from a typical tap-water EC of 0.3-0.8 mS/cm down to 0.01-0.05 mS/cm. From that blank slate, every mineral your plant receives comes from you, not from the municipal water supply.
What RO removes from your water
| Contaminant | Why it matters for hydroponics |
|---|---|
| Chlorine and chloramine | Damages beneficial microbes; can affect root health |
| Heavy metals (lead, copper) | Toxic at low concentrations; disrupts enzyme function |
| Excess calcium and magnesium | Raises your baseline EC; interferes with custom feeding ratios |
| Sodium | Competes with potassium uptake; causes salt stress |
| Nitrates | Skews your N-P-K ratio before you add a drop of nutrients |
| Pesticide residues | Can accumulate in the root zone; long-term stress |
Source: Greenhouse Product News – RO systems in commercial hydroponics
When RO water matters most
RO water gives you the biggest payoff in three situations:
- Hard water areas. If your tap water EC is above 0.4 mS/cm, you are working with a contaminated baseline. Many city water supplies run 0.5-0.8 mS/cm – that is EC you did not plan for.
- Sensitive crops. Ferns, strawberries, and leafy greens harvested before flowering are especially sensitive to excess minerals and salts. RO water removes the guesswork.
- Commercial or multi-plant systems. In commercial hydroponics, RO filtration is considered standard practice because inconsistent source water creates inconsistent results across hundreds of plants.
RO Water Benefits vs. Tap Water for Hydroponics
| Factor | RO Water | Tap Water |
|---|---|---|
| Starting EC | 0.01-0.05 mS/cm | 0.2-0.8 mS/cm (varies by region) |
| Chlorine | Removed (>99%) | Present (damages microbes) |
| Heavy metals | Removed | Variable (depends on pipes) |
| pH predictability | High – stable base | Low – fluctuates with mineral content |
| Nutrient accuracy | Exact – you control what goes in | Approximate – minerals already present |
| Cost | System cost + ~$0.01-0.05 per litre | Tap rate only |
| Remineralization needed? | Yes – add Cal-Mag after filtering | No – minerals already present |
The one trade-off with RO water is remineralization. Because RO strips calcium and magnesium along with everything else, you need to add them back. Most hydroponic nutrient lines include a Cal-Mag supplement for exactly this reason. Add it first, before your base nutrients, so it dissolves cleanly.
One grower on Overgrow.com described the shift after switching to a fully controlled setup:
“My flood and drain setup is fully automated. I don’t have to do much, just put the plant in the tent and hit go.”
Starting with RO water is what makes that level of predictability possible. When your source water is consistent, everything downstream – nutrients, pH, EC – becomes easier to manage and automate.
How to Set Up an RO System with Your Hydroponic Setup
Setting up RO water for hydroponics is a one-time installation that pays off every grow. Here is the process from first connection to ready-to-use nutrient water.
What you need
- An RO unit (50-100 GPD is enough for most home setups; larger systems need 150-200 GPD)
- A storage tank or reservoir (RO systems produce water slowly – 50 GPD fills roughly 190 litres per day)
- A TDS or EC meter to verify your output water quality
- Cal-Mag supplement
- Your standard nutrient line
Step-by-step setup
Step 1 – Choose your RO unit location.
Mount it under the sink or near your reservoir. RO systems need a cold water supply line and a drain line for the waste water they reject (typically 3-4 litres of waste per 1 litre of filtered water for home units).
Step 2 – Connect the supply line.
Use the saddle valve that comes with most RO kits to tap into your cold water line. Do not use hot water – heat damages the membrane.
Step 3 – Route the output to your storage tank.
Let the unit fill the tank before your first grow. A 50 GPD unit fills a 100-litre tank in about 10-12 hours. Most growers run it overnight so the reservoir is ready by morning.
Step 4 – Verify your output EC.
Before adding any nutrients, test the RO water with your EC or TDS meter. Target: below 0.05 mS/cm (50 ppm on a 500 scale). If the reading is higher, the membrane may need flushing or replacement.
Step 5 – Add Cal-Mag first.
Pour your Cal-Mag dose directly into the RO water and let it mix for a few minutes before adding base nutrients. This prevents precipitation and gives the plants the calcium and magnesium the RO process removed.
Step 6 – Add your base nutrients and check EC.
Follow your nutrient line’s feed chart for your current growth stage. Because your starting EC is near zero, the final EC reading will be accurate and you will hit your target range precisely.
Step 7 – Adjust pH.
RO water has little buffering capacity, which means pH moves quickly with small doses of pH up or pH down. Add your pH adjuster in small amounts and test frequently. Target pH ranges:
| System type | Target pH |
|---|---|
| DWC / NFT / Aeroponics | 5.5-6.0 |
| Coco coir | 5.8-6.2 |
| Rockwool | 5.5-6.0 |
| Ebb and flow | 5.8-6.3 |
Step 8 – Monitor and top up.
As plants drink, the reservoir EC and pH will drift. RO water systems do not stop this drift – they only give you a clean starting point. Monitoring between top-ups is still essential, especially in warm conditions or during heavy feeding stages.
EC Targets by Growth Stage (RO Water Base)
Because RO water starts at near-zero EC, the following targets represent your nutrient-added EC after mixing. These numbers are based on the General Hydroponics Flora Series medium feed program, the most common home grower nutrient line.
| Growth Stage | EC Range (mS/cm) | PPM (500 scale) |
|---|---|---|
| Seedling / Clone | 0.4-0.8 | 200-400 |
| Early Veg | 1.0-1.6 | 500-800 |
| Late Veg | 1.5-2.1 | 750-1050 |
| Early Flower | 1.5-2.0 | 750-1000 |
| Mid Flower | 1.5-2.0 | 750-1000 |
| Late Flower | 1.0-1.4 | 500-700 |
| Flush | Below 0.1 | Below 50 |
Source: General Hydroponics Flora Series feed chart (medium feed program)
Important: EC targets vary significantly by nutrient brand and program. Commercial programs like Athena Pro run EC at 3.0 mS/cm across veg and flower – nearly double the GH medium program. If you are following a different nutrient line, use its feed chart. The table above is a verified baseline for home growers using GH Flora Series.
How to Choose an RO System for Hydroponics
Not all RO units are the same. Here is what to look at when selecting a system for a hydroponic grow.
Flow rate (GPD – gallons per day)
GPD tells you how much filtered water the unit can produce in 24 hours.
| Setup size | Recommended GPD |
|---|---|
| 1-4 plants, small reservoir | 50-75 GPD |
| 5-12 plants, medium system | 100-150 GPD |
| 13+ plants, large system | 200+ GPD |
Note: Most home RO units are rated under ideal pressure (60 psi). At lower pressures, real output may be 30-50% lower. If your home water pressure is below 50 psi, look for a unit with a booster pump or buy a higher GPD rating than you think you need.
TDS reduction rate
Look for membranes rated at 95% TDS (total dissolved solids) reduction or higher. A unit that removes 90% TDS on water starting at EC 0.6 will leave you with EC 0.06 – workable, but not ideal. At 98% reduction, the same water comes out at EC 0.012 – a clean base.
Filter stages
Most hydroponic-grade RO units are 4-5 stage:
- Sediment pre-filter (removes particles, protects the membrane)
- Carbon block pre-filter (removes chlorine and chloramine)
- RO membrane (removes dissolved solids)
- Post-carbon filter (polishes taste/odour)
- (Optional) Remineralization or DI stage
The remineralization stage is useful if you want to skip the manual Cal-Mag step, but it adds minerals in fixed ratios – you have less control over the exact amounts. Most hydroponic growers skip this stage and add Cal-Mag manually so they can adjust the dose by growth stage.
Waste water ratio
Home RO units typically produce 3-4 litres of waste water for every 1 litre of filtered water. This is called the waste ratio or reject ratio. For a grower topping up a 100-litre reservoir weekly, that means 300-400 litres of waste water going down the drain. Commercial systems and high-efficiency membranes can get this down to 1:1. If water cost or conservation matters to you, look for a low-waste or permeate pump unit.
Why Manual Monitoring Falls Short with RO Water
RO water creates an excellent starting point, but it does not freeze your nutrient solution in place. pH and EC continue to move after mixing for several reasons.
pH moves quickly in RO water. Because RO strips most buffering minerals, there is little resistance to pH change. A plant drinking actively, CO2 dissolving in the water, or a small temperature swing can shift pH by 0.3-0.5 points overnight. Growers who check pH once a day are often checking after it has already drifted out of range and sat there for hours.
EC rises as water evaporates. Plants drink water faster than they drink nutrients. As water level drops, the nutrient concentration in what remains goes up. Your EC climbs. If you top up with plain RO water, the EC drops again. This cycle creates instability if left unmanaged.
Manual checks miss the gaps. A twice-daily check catches the drift after the fact. Between your morning reading and your evening reading, pH may have spiked and crashed and recovered – and your plants experienced all of it.
One grower on 420magazine.com explained why they moved away from a DIY approach:
“I had made a DIY pH doser and I keep running into lockout issues. That seems to be an issue of the past now that I am 2 weeks into my grow with the Growee system.”
That transition – from reactive manual checking to automated correction – is what turns a good RO setup into a reliable one.
Automating pH and EC with RO Water
Once you have RO water as your base, the next upgrade is automating what happens to it after mixing. This is where Growee’s pH and feeding automation fits in.
The Growee Starter Combo connects to your reservoir and monitors pH and EC continuously. When pH drifts out of your target range, it doses pH up or pH down automatically – in small, precise amounts – and logs the correction in the app. When you need to top up nutrients, it manages EC dosing to keep your solution consistent.
Because RO water has low buffering capacity, the small automated doses Growee delivers are enough to correct drift without overshooting. Manual dosing into unbuffered RO water is more prone to overshoot because there is nothing to slow the pH swing between drops.
“I have 2 of these systems… I was able to take a one week cruise and felt comfortable letting the Growees do their thing while I was away. I came home to some beautiful plants with surprising growth and absolutely no nutrient burn or deficiencies!”
Dog – pH Automation
RO water removes the guesswork from your source. Growee removes the guesswork from what happens next.
The Bottom Line
RO water for hydroponics gives you a near-pure, predictable base for your nutrient solution by stripping chlorine, heavy metals, and excess minerals from tap water before you add a single drop of nutrients. Set up your RO unit, verify the output EC is below 0.05 mS/cm, add Cal-Mag first, then build your nutrient solution from there. The result is more accurate feeding, more stable pH, and fewer mystery deficiencies. If you want that stability maintained automatically between checks, a Growee pH controller monitors and corrects your reservoir 24 hours a day so you never come back to a plant that has been sitting in out-of-range water.
