Water Quality for Growers: Tap Water, Hard Water and RO

Water is the one ingredient in your reservoir you did not choose. Everything else — the base feed, the additives, the pH adjuster — goes in on top of whatever came out of the tap, and in a 30-litre tank the water is around 29.9 litres of it. Yet almost nobody measures it. Two growers can follow the same feed chart to the millilitre, one in Manchester and one in Ipswich, and get completely different plants, because one started at 40 ppm and the other started at 380. This guide covers what is actually in UK tap water, how to find out what is in yours, when a carbon filter is enough, when reverse osmosis is worth the money and the drain water, and — the part people skip — what you have to put back once you have taken everything out.

Why the water is the biggest thing in your tank

When you mix a feed to EC 1.8, you are aiming at a total dissolved solids figure. Your meter cannot tell the difference between a milligram of calcium that came out of a chalk aquifer and a milligram of calcium that came out of a bottle of base nutrient. It reads the total. So if your tap water is already sitting at EC 0.6, you have used a third of your budget before you have opened anything.

That is the mechanical problem, and it is easy to correct once you know the number: measure the tap, subtract it, and mix to the difference. The harder problem is that the dissolved solids in tap water are not the ones your plant is short of. Hard water is mostly calcium and magnesium carbonates and bicarbonates, picked up from chalk and limestone. Those carbonates give the water alkalinity — a chemical buffering capacity that resists pH change. It is why some growers can add pH Down all evening and watch the reading crawl back up by morning, while others move a whole point with a couple of millilitres.

Meanwhile the things that actually vary in the water — sodium, chloride, sulphate, nitrate — accumulate. In coco and in soil they build up in the substrate over weeks, because you keep adding them with every feed and the plant only takes what it needs. That is one of the reasons run-off EC climbs even when your input EC stays put.

What is actually in UK tap water

Mains water in the UK is treated to be safe to drink, which is a completely different specification from being ideal to grow in. Four things in it matter to a grower.

Hardness — calcium and magnesium carbonates

This is the big one, and it varies enormously across the country. Broadly, water drawn from chalk and limestone aquifers in the south and east is hard; water from upland reservoirs over granite and peat in Scotland, Wales, the north west and the south west is soft. Water companies report hardness as milligrams per litre of calcium carbonate, and the conventional UK bands run like this.

Classification bands vary slightly between water companies — check the figure your own supplier publishes rather than assuming from the region.

Chlorine and chloramine

Both are disinfectants, added deliberately so the water stays safe on its way to you. Chlorine is volatile and will gas off if water is left standing. Chloramine — chlorine bonded to ammonia — will not. It is used precisely because it persists, and many UK suppliers have moved to it for that reason. This matters if you are running any kind of biological product, because both compounds are designed to kill microbial life and cannot tell your root-zone inoculant from anything else.

Nitrate

Common in agricultural catchments and legally permitted up to a limit. It is a plant nutrient, so it is not harmful as such, but it is nitrogen you did not add and did not count. If you are running a low-nitrogen regime it is worth knowing about.

Sediment, iron and organics

Older mains and long service pipes carry rust and particulate. It rarely harms plants directly, but it blocks drippers, coats sensors and shortens the life of anything with a fine aperture.

Measure your own water before you change anything

The published report tells you the average for your zone. Your kitchen tap tells you the truth on the day. The two are usually close, but supply zones get switched and blended, and a report is a yearly summary.

An EC meter reads electrical conductivity — how easily current passes through the water, which rises with the amount of dissolved salt in it. Run the tap for thirty seconds to clear the standing water in the pipe, fill a clean jug, let it come to room temperature, and read it. That number is your baseline, and everything else in this guide follows from it.

What to measure with

If you want the whole measuring and correcting job in one box rather than assembled piecemeal, the shop's own kit bundles a Truncheon, a pH stick, pH Up and Down, a dechlorinator and dosing syringes together.

What your tap reading actually means

Here is the practical decision, based on the EC or ppm you just read at the tap.

Note that the problem is not only the height of the number. It is that you cannot see inside it. Two supplies can both read 350 ppm — one mostly calcium carbonate, one mostly sodium chloride. The first is inconvenient; the second is actively hostile, because sodium competes with potassium at the root and chloride accumulates. The water report tells you which you have.

Chlorine, chloramine and carbon filtration

If you do only one thing to your water, do this one. It is cheap, it is quick, and it removes the disinfectants that are working directly against the biology you are trying to build in the root zone.

The old advice was to fill your tank and leave it overnight so the chlorine gasses off. That still works for chlorine. It does not work for chloramine, which is stable and will sit there indefinitely — and a growing number of UK supplies are chloraminated specifically because it persists in the network. Standing water overnight also warms it up and lets it stagnate, which is not a free lunch either.

There are two proper answers. A liquid conditioner deactivates chlorine and chloramine on contact, costs very little per tank, and does not change your EC. A carbon filter plumbed between the tap and the reservoir removes chlorine along with pesticides, herbicides and volatile organic compounds, and needs no power — it runs on mains pressure alone. The filter is the better answer if you are filling regularly; the conditioner is the better answer if you fill occasionally or want a belt-and-braces step before adding a microbial product.

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Reverse osmosis: what it does and when you need it

A carbon filter takes out contaminants but leaves the dissolved minerals — the calcium, magnesium and sodium are still there, and so is your starting EC. Reverse osmosis is the step that removes those too. Mains pressure pushes water through a semi-permeable membrane whose pores pass water molecules and reject dissolved salts, which are flushed to the drain. What comes out the other side is close to blank: typically 98-99% of dissolved contaminants removed, and with an added de-ionisation stage you can get to a true zero reading.

The case for it is straightforward. If your tap water is very hard, RO hands you back control of three things at once: your EC becomes entirely yours to set, your pH stops being buffered by carbonate and moves predictably, and the sodium and chloride that were accumulating in your substrate stop arriving. A feed recipe that works keeps working, because the starting point does not drift.

The case against it is equally straightforward, and it is covered in the next section. RO is not free, it is not instant, and it creates a new job for you.

What RO actually costs you

Three things, and none of them is the purchase price.

Water down the drain

RO separates your supply into product water and concentrate. A typical grow-room unit runs somewhere around a 2:1 waste-to-product ratio, so producing 50 litres for your reservoir sends roughly 100 litres to the drain. That is not wasted in any environmental sense — it goes back into the sewer like any other water — but if you are on a meter it appears on the bill. Many growers catch the concentrate and use it on the garden, where a slightly elevated mineral content is neither here nor there.

Time

RO is slow. A 75 GPD unit produces roughly 284 litres a day, which sounds ample until you realise it is about 12 litres an hour. Filling a 100-litre reservoir is an eight-hour job you have to plan the evening before, not something you do while the lights are off and you are already in the tent. This is the single most common complaint from people who buy one, and it is entirely solved by having a holding tank and filling it in advance.

Pressure

Membranes are specified for a pressure range, commonly around 40-80 psi. Below that, output falls away sharply and rejection gets worse — you get less water and it is less pure. If your mains pressure is weak, particularly in a flat or at the end of a long run, a booster pump is not an accessory, it is the difference between the unit working and not.

RO water is empty — and that is a job, not a finish line

This is the step that catches people out, and it produces a very recognisable failure: someone installs RO, is delighted with their 0 ppm baseline, mixes exactly to the feed chart, and watches their plants develop deficiency symptoms within a fortnight.

The reason is that most feed charts are written on an assumption about your water. Hard tap water supplies a meaningful quantity of calcium and magnesium for free, and the manufacturer has quietly counted on it. Take it away and the chart is short of both. Calcium is immobile in the plant, so a shortage shows in the new growth at the top rather than the old leaves at the bottom, and magnesium shortage shows as interveinal yellowing on older leaves — a pattern people routinely misread as a nitrogen problem and try to fix by feeding harder, which makes it worse.

The fix is simple. Add a calcium-magnesium supplement to RO or very soft water, first, before anything else goes in. Add it to the water and stir before the base nutrient, because calcium and certain other elements will react and precipitate out as solids if they meet each other at high concentration — that is the white sludge in the bottom of the tank, and it is nutrient that is no longer available to anything.

Or use a base feed built for your water

Some nutrient lines sidestep the problem by selling two versions of the same product — one formulated for soft or RO water with the calcium and magnesium already in it, and one for hard water with less of both, on the assumption your supply is bringing them. If you know which water you have, buying the matching version is tidier than correcting afterwards.

Blending: the answer most people actually want

You do not have to choose between raw tap water and pure RO. Blending the two gives you any starting point you like, and for most home growers it is the sensible middle.

The arithmetic is simple, because EC blends more or less linearly by volume. If your tap reads 0.7 EC and your RO reads 0.0, then a 50/50 mix reads about 0.35, and a 25% tap / 75% RO mix reads about 0.18. Pick the starting point you want, work out the ratio, and mark the levels on the side of the tank so you are not measuring every time.

The advantages are real. You halve your RO running time and drain water. You keep some of the calcium and magnesium that were arriving free, so you need less CalMag. And you keep a little buffering capacity, which — counter-intuitively — makes pH easier to hold steady rather than harder. Water with essentially no alkalinity is very easy to move and therefore very easy to overshoot; a couple of drops of pH Down in pure RO can swing the reading a long way.

Water temperature, storage and dissolved oxygen

Having got the chemistry right, do not let the physics undo it. Water temperature controls how much oxygen the water can hold, and roots need oxygen at least as much as they need feed. Cold water holds more dissolved oxygen than warm; warm water holds less and simultaneously suits root pathogens better. Somewhere in the high teens to low twenties Celsius is the usual target for a reservoir — cool enough to stay oxygenated, warm enough not to shock the root zone.

In a tent in summer this is genuinely difficult, because the reservoir is sitting in a warm room under a light. In winter the opposite problem appears: a tank in a garage can drop into single figures, and cold water slows root uptake noticeably. A small aquarium heater fixes the second problem for very little money; a chiller fixes the first, at rather more.

Two more storage points. Keep stored water covered and out of the light — an open tank in a lit room grows algae, which consumes oxygen and clogs everything. And do not store nutrient solution indefinitely; mixed feed does not keep. Plain filtered or RO water stores perfectly well, so if you are filling in advance, store the water and mix on the day.

If summer heat is your problem more broadly, our guide to keeping a tent cool deals with the room as well as the tank.

Six mistakes worth avoiding

1. Never measuring the tap at all. Everything in this guide follows from one number that takes thirty seconds to read. Mixing to EC 1.8 means something completely different on 40 ppm water and on 400 ppm water.
2. Leaving water to stand and assuming it is dechlorinated. That works for chlorine. If your supply is chloraminated it does nothing at all, and you have warm stagnant water for your trouble.
3. Going to RO and not adding calcium and magnesium back. The most common and most avoidable RO failure. Pure water is a blank canvas, and a blank canvas has no calcium in it.
4. Adding CalMag after the base nutrient, or straight into concentrate. Calcium precipitates when it meets concentrated feed. Water first, CalMag second, base feed third, pH last.
5. Ignoring the RO pre-filter until the membrane dies. The carbon pre-filter exists to stop chlorine reaching the membrane. Skip the cheap replacement and you buy the expensive one.
6. Chasing pH on very hard water. If the reading crawls back up every morning you are not doing it wrong — carbonate alkalinity is pushing it back. Reduce the alkalinity by filtering or blending rather than pouring in more acid.

Two of these connect directly to problems covered elsewhere: our pH guide explains the buffering behaviour in full, and our nutrient burn guide covers what happens when the total EC ends up higher than you intended.

Quick answers

How do I find out how hard my water is?

Your water company publishes a quality report by postcode on its website, free and without registration. It will give hardness in mg/l of calcium carbonate, often alongside degrees Clark, plus nitrate, sodium and chloride figures and whether your supply is treated with chlorine or chloramine. That takes about two minutes and is more informative than any general regional map. For the day-to-day number, an EC meter at the tap tells you what is actually arriving.

Is EC the same as ppm?

No, and this causes more confusion than almost anything else in growing. EC is a direct measurement of conductivity in millisiemens per centimetre. ppm is a conversion from EC using an assumed factor, and two factors are in common use: on the 500 scale EC 1.0 shows as 500 ppm, and on the 700 scale the identical water shows as 700 ppm. Always check which scale a meter or a feed chart is using before comparing numbers, or work in EC and avoid the problem.

Will leaving water out overnight remove the chlorine?

It will remove chlorine, given enough time and surface area, because chlorine is volatile. It will not remove chloramine, which is chlorine bonded to ammonia and is specifically used because it stays in the water. Many UK supplies are chloraminated. Standing water also warms up and stagnates, so if you are running biological products a conditioner that treats both compounds, or a carbon filter, is a far more reliable answer.

Do I really need RO, or is a carbon filter enough?

It depends entirely on your starting EC. A carbon filter removes chlorine, pesticides and organic contaminants but leaves the dissolved minerals, so your EC barely changes. If your tap is soft or moderate, that is all you need. If your tap is over about 0.6 EC, the minerals themselves are the problem — they are eating your EC headroom and buffering your pH — and only RO or blending fixes that. Measure first, then decide.

Why did my plants get worse after I installed RO?

Almost certainly a calcium and magnesium shortage. Most feed charts assume your water is contributing some of both, and hard tap water quietly does. Pure RO contributes none, so a chart followed exactly is now short. Add a CalMag supplement to the water before the base nutrient, or switch to the soft-water version of your nutrient line, and the problem usually resolves within a couple of feeds.

How much water does an RO unit waste?

A typical grow-room unit runs around a 2:1 waste-to-product ratio, so roughly two litres to the drain for every litre you keep. Ratios vary by unit and get worse if your incoming pressure is low, which is one of the arguments for a booster pump. The concentrate is not contaminated in any meaningful sense — it is just your tap water with the minerals concentrated — so plenty of growers catch it and use it on the garden or for cleaning.

Can I use rainwater instead?

Rainwater is naturally very soft and very low in dissolved minerals, so chemically it behaves much like RO — including needing calcium and magnesium added back. The catch is that it is not sterile and not clean. Collected off a roof it carries organic debris, bird droppings, algae and whatever was on the tiles, and a butt in summer is a warm dark tank full of biology. If you use it, filter it, keep the butt covered and cool, and check its EC — a neglected water butt can read surprisingly high.

What order should I mix everything in?

Fill with your water first, at roughly the temperature you intend to feed at. Treat it or filter it. Add calcium and magnesium if you are on RO or soft water, and stir. Add your base nutrient — part A fully dispersed before part B, never the two together. Add any additives. Then, last of all, check and adjust pH, because every one of those additions moves it. Checking pH before the nutrients are in tells you nothing useful.