Reverse Osmosis (RO) Water for Growing: Do You Need a Filter?

Reverse osmosis (RO) filters tap water until almost nothing is left in it but water. Growers use it for a clean, low-EC starting point, most often because their tap water is hard. This guide covers how RO works, the RO filters we sell, what to add back, and when you do not need one.

What is reverse osmosis?

Osmosis is the natural movement of water through a membrane towards the saltier side. Reverse osmosis uses pressure to push it the other way: tap water is forced against a membrane so fine that water gets through and most of what is dissolved in it does not. The held-back salts leave in a stream of waste water, so an RO filter needs a drain as well as a tap, and some of the water you put in always goes down it.

The pages for our RO filters describe the result as "low EC, nearly pure water", with up to 95% of salts, heavy metals, chlorine, chloramines, nitrates and nitrites removed, along with sediment, herbicides and pesticides.

Why growers use RO water

Hard water. Hard tap water carries a lot of dissolved calcium and magnesium. It shows up as EC before any feed goes in, and tends to push the pH up. RO takes most of that background EC out.
A known starting point. Tap water differs from one area to the next. RO water starts from a low EC every time, so what the meter reads is the feed you added.
Microbes. The pages say taking out the chlorine lets soil life flourish, especially if you use bacterial or microbial additives.
Humidifiers. The G.A.S SonicAir Pro page says it must be run on RO water to avoid damage to its ceramic discs, and that anything else voids the warranty; the Ram humidifiers page asks for RO water only.

The RO filters we sell

All three are 3-stage filters made by Growmax. The pages give the hourly outputs as "up to", so treat them as best-case figures.

Growmax reverse osmosis filters, as their product pages describe them.
RO filter	Litres per hour	Litres per day	Filters last (around)
Power Grow 500	Up to 20	Around 500	12,000 litres or 6 months
Mega Grow 1000	Up to 40	Around 1,000	12,000 litres or 6 months
Growmax 3000	Up to 125	Up to 3,000	40,000 litres or 6 months

Choose by how fast you need the water. At up to 20 litres an hour, the Power Grow 500 fills a large tank slowly, so it pays to filter into a storage tank such as a Flexitank ahead of time. The two bigger filters fill a tank faster.

Low water pressure. Growmax RO filters are designed to work at 40-60 psi. Below that, the Growmax RO Booster Pump will normally bring the supply into range (it has a two-pin European plug, so you need an adapter). The High Flow Booster Pump is designed for the Mega Grow 1000. Not sure which pump your filter needs? Call us before you order.
Filling a tank. The Growmax RO Float Valve Kit fits to the tank or reservoir where the filtered water is stored and keeps it topped up, so there is always some ready. It works with every RO and water filter we sell.

What to add back: cal-mag

RO takes out most of the calcium and magnesium along with the other salts, and plants need both. So with RO water a cal-mag product goes in before the base feed: the Athena CaMg page recommends bringing the EC up to around 0.3 or 0.4 before adding any other product. Canna CALMAG Agent, Mills CalMag and Biobizz CalMag are for soft or RO water too; our cal-mag guide compares all eight we sell and lists the feeds that need none.

Then add the base feed and the other additives, and adjust the pH last (see our pH and EC meters guide). If your feed comes in hard and soft water versions, use the soft one with RO water: the T.A. TriPart page says so for its Micro.

Do you need an RO filter? What to do

Measure your tap water with an EC meter such as the Bluelab Truncheon, before anything is added. The figures below are EC in mS/cm; CF is ten times the EC.
Under 0.4: Canna count that as soft water. There is little hardness for RO to take out - soft water usually calls for cal-mag, not a filter.
0.4-0.7: the base EC Canna say their UK nutrients are designed for. With their feeds, no RO needed. Canna's Aqua chart is written for softer water still (EC 0.2) - see the Canna Aqua feed chart. With another brand, check its feed chart or ask us.
Higher: first see whether your feed has a hard water version. Canna Hydro, T.A. TriPart Micro, Mills Basis A and Vitalink Hydro Max do; the TriPart page puts hard water at more than 70 mg/L of calcium (your water company's hardness report for your postcode gives this - read the calcium figure, not calcium carbonate). If your feed does not, or you want full control of what goes in the tank, choose an RO filter.

Only want rid of chlorine? You do not need RO. The 2-stage Growmax filters - Eco Grow 240, Garden Grow 480, Super Grow 800 and Pro Grow 2000 - have no RO membrane, so they do not lower the EC, but deliver chlorine-free water far faster: the number in each name is its approximate litres per hour. If your supplier uses chloramine, ask us before choosing a filter, or treat the water instead: Ecothrive Neutralise is for both chlorine and chloramine (Ecothrive recommend 1 drop per litre), and so is Dr Hydro Dechlorinator, at the rate on the label.

Looking after an RO filter

The product pages give two jobs: change the filters at the interval in the table, and the membrane about every 9 months. Replacement filters and membranes are listed in the drop-down on each filter's page.

Quick answers

What is reverse osmosis (RO) water?

Tap water forced through a very fine membrane that holds back most of what is dissolved in it. What comes out is low in EC and close to pure water.

Do I need an RO filter?

Not if your tap water's EC is within the range your feed is made for, or the feed has a hard water version. RO earns its place when the water is hard and the feed has no hard water version, or when a humidifier asks for it.

Do I need cal-mag with RO water?

With most feeds, yes: RO takes out most of the calcium and magnesium plants need. Cal-mag goes in before the feed, to the EC its maker gives - the Athena CaMg page says around 0.3 or 0.4. See all our cal-mag additives.

Can I mix RO water with tap water?

Growers commonly do, blending a little tap water back in to reach the base EC their feed is designed for. Measure the mix with an EC meter - ours are in Nutrient Management.

More Growers' Guides

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.