Short version? If you're on RO or soft water, get CANNA Calmag Agent. It's £12.95 a litre and you only add 0.1 to 0.5ml per litre, just until the water reads EC 0.4. Organic grower? BioBizz Cal Mag starts at £4.49 for a 250ml tester. Leaning hard on PK boosters in flower? Plant Magic MAGNE Cal+ is made for that. And if your tap water is already hard, you might not need any of it. Check the EC first.
The best CalMag supplements
CANNA Calmag Agent
Less a supplement, more a water fixer. CANNA make all their feeds for normal water with a calcium and magnesium background EC of 0.4 to 0.7. Calmag Agent tops soft or RO water back up to that. Then your feed goes in as usual.
- 0.1 to 0.5ml per litre
- Target EC 0.4
- RO and soft water
- All systems
- Tiny dose. The 1L treats somewhere between 2,000 and 10,000 litres of water, depending how soft yours is
- Goes in first, before any other CANNA nutrients, so it slots into a programme like CANNA Coco A&B without fuss
Plant Magic MAGNE Cal+
This one's aimed at flower. Plant Magic's feed chart has it in at 0.5 to 1ml per litre for the whole of the flowering stage, as part of your normal feed.
- 0.5 to 1ml per litre
- Through flowering
- Any system or media
- 500ml to 10L
- Plant Magic say it helps PK boosters do their job, especially their own Platinum PK 9/18 and PK 13/14
- Can go on plants already showing a CalMag problem, to help sort it quicker
BioBizz Cal Mag
Fully organic and BioBizz certified. Their feeds are made for medium-hard water, so if you're on RO or in a soft water area, this is what gets you back to where they expect you to be.
- Fully organic
- 0.5 to 1ml per litre hydro and coco
- 0.3 to 0.8ml per litre soil
- 250ml to 10L
- Separate doses for prevention and for fixing a deficiency that's already showing, up to 1.4ml per litre in hydro and coco
- The 250ml is £4.49. Cheapest way into CalMag on this list
Plagron Calmag Pro
Plagron's answer to the RO problem. Calcium and magnesium in one bottle, in a ratio meant to stop the two competing with each other. Plagron call it the base you build your nutrient water on.
- Up to 1ml per litre
- About 0.75ml from RO
- Every watering
- All substrates
- Some calcium and magnesium feeds won't mix because of sulphates. This one has both, already balanced
- Commonly run with Plagron Cocos A&B when growing coco on RO water
Advanced Nutrients Sensi Cal Mag Xtra
The fancy one. Chelated calcium, magnesium and iron, plus 20 L-form amino acids. Advanced Nutrients built it for hydro systems, for veg and flower, at 2ml per litre.
- 2ml per litre
- Veg and flower
- Hydro systems
- Foliar once a week
- Doubles as a weekly foliar spray at the same 2ml per litre
- Going organic? There's a separate OG Organics Sensi Cal Mag Xtra from £25.95
Shogun Calmag
Shogun don't see CalMag as a repair job. They pitch it for moving nutrients round the plant quicker, with a bit of iron and nitrogen in there too.
- 1 to 2ml per litre
- Foliar 10 to 15ml per litre
- Soil, coco and hydro
- 250ml to 25L
- Proper foliar instructions, once a week to correct imbalances. At 10ml per litre in water with a zero background, Shogun say the spray should read EC 2.4
- Made to pair with Shogun PK Warrior 9/18 in flower
Do you actually need CalMag?
Only if your water is short of it. And that's a meter reading, not a guess.
CANNA give the clearest numbers. Their feeds are designed for water with a calcium and magnesium background EC of 0.4 to 0.7.
- Below 0.4. Soft water or RO. You need CalMag, or you're asking for deficiencies.
- 0.4 to 0.7. You're fine. Keep your money.
- Above 0.8. CANNA suggest a reverse osmosis unit to strip it right down, then CalMag to bring it back up to 0.4.
Funny old business, that last one. Pay to take the minerals out, then pay a bit more to put the right ones back. But it's the only way you know exactly what's in your tank.
Then there's the order. CANNA and Plagron treat CalMag as step one, into the water before anything else. Advanced Nutrients go the other way, base nutrients first and additives after. So follow the label on the bottle in your hand.
You'll need a meter for all this. Our pH and EC meter guide has the ones we'd buy, or read our closer look at the Bluelab Truncheon if you only need EC.
At a glance
Bottle prices don't say much on their own, because the doses vary so much. So we've worked out roughly what each one costs per 100 litres of feed.
| Supplement | Dose | Per 100L of feed | Price |
|---|---|---|---|
 CANNA Calmag Agent |
0.1 to 0.5ml per L | 13p to 65p (1L) | From £12.95 |
 Plant Magic MAGNE Cal+ |
0.5 to 1ml per L | 85p to £1.70 (1L) | From £8.90 |
 BioBizz Cal Mag |
0.5 to 1ml per L (hydro, coco) | 67p to £1.35 (1L) | From £4.49 |
 Plagron Calmag Pro |
0.75 to 1ml per L | £1.20 to £1.60 (1L) | From £16.00 |
 AN Sensi Cal Mag Xtra |
2ml per L | About £4.80 (1L) | From £11.00 |
 Shogun Calmag |
1 to 2ml per L | £1.25 to £2.50 (1L) | From £5.49 |
Prices as at November 2026. Cost per 100L is worked out from the label dose and the bottle size in brackets. CANNA's dose depends on how soft your water is, so the low end assumes water that's nearly there already. Check the product page for the live price.
How to choose
- On RO or very soft water, any feed? CANNA Calmag Agent. Add it till the meter says 0.4 and get on with your day.
- Growing organic or running BioBizz? BioBizz Cal Mag.
- Coco on RO with Plagron feeds? Plagron Calmag Pro.
- Pushing PK boosters in flower? Plant Magic MAGNE Cal+, or Shogun Calmag if you're on PK Warrior.
- Hydro, and you want the amino acids and iron? Advanced Nutrients Sensi Cal Mag Xtra.
- Running Mills, Dutch Pro or BAC? Stick with the house CalMag: Mills Nutrients Cal Mag (from £24.00), Dutch Pro Cal Mag (from £19.50) or BAC CalMag 2.0 (from £8.00).
Adding silica as well? Our silica supplement guide covers the mixing order for that.
Quick answers
Do I need CalMag if I use tap water?
Only if your tap water is soft. CANNA design their feeds for water with a calcium and magnesium background EC of 0.4 to 0.7. If your tap water reads below 0.4, add CalMag. If it's already in that range, you probably don't need it.
What EC should my water be after adding CalMag?
Around 0.4. CANNA say add Calmag Agent gradually, at 0.1 to 0.5ml per litre, until soft or RO water reaches EC 0.4. Then add your nutrients as normal.
Do I need CalMag with RO water?
Yes. RO water has almost nothing left in it, calcium and magnesium included, so you need to put them back before you feed. That's the main job of CANNA Calmag Agent, Plagron Calmag Pro and BioBizz Cal Mag.
Should CalMag go in before or after nutrients?
Usually before, but check your label. CANNA and Plagron treat it as the first thing into the water. Advanced Nutrients say add your base nutrients first, then Sensi Cal Mag Xtra and other additives, then adjust the pH.
Can I use CalMag with PK boosters?
Yes, and some are made for it. Plant Magic say MAGNE Cal+ helps PK boosters like Platinum PK 9/18 and PK 13/14 work better, and Shogun pair their Calmag with PK Warrior 9/18. BioBizz note that calcium and magnesium can become less available during heavy PK feeding.
Can you foliar spray CalMag?
Yes, with some of them. Shogun Calmag goes on at 10 to 15ml per litre once a week to correct imbalances. Advanced Nutrients Sensi Cal Mag Xtra is 2ml per litre, once a week.
Still stuck? Give us a ring on 020 8686 9643 or email info@thehydrobros.com.
Or pop into the shop for a natter: Unit 12, Progress Business Park, Progress Way, Croydon CR0 4XD.
More grow guides
- Best silica supplements
- Rock Resinator: a closer look
- CANNA Coco A&B: a closer look
- Best coco coir and coco mixes
- Best pH and EC meters
- How to read NPK ratios
The Hydro Bros Team. We sell this kit and talk growers through it every day in our Croydon shop.
