Before you touch a bottle of nutrient, your water has already made some decisions for you. Tap water varies a lot across New Zealand soft in parts of the South Island, moderately hard in parts of the North. Whatever's coming out of your tap is mixing with your carefully calibrated nutrients whether you account for it or not.
The two numbers that matter most are EC (how many dissolved minerals are already in your water before you add anything) and pH (your actual starting point, before you adjust it). Test your tap water before your first grow. Most growers skip this step and spend weeks chasing a symptom that traces straight back to the tap. A Bluelab pH & EC Starter Pack covers both readings in one go, avoid the cheap meters they're rarely accurate enough to trust.
Hard water vs. soft water
Hard water carries elevated calcium and magnesium carbonate. Those are useful minerals, but the carbonate also raises pH and buffers against adjustment, so you'll go through more pH-down to hit 5.5–6.5, and the reading tends to creep back up faster than you'd like. If your pH keeps drifting upward after you've already dialled it in, this is usually why.
Soft water is the opposite problem. Low mineral content means little buffering, so pH is easy to adjust — but because you're starting from a more neutral base, Cal-Mag has to come entirely from your nutrient line. Plants on soft water can show Cal-Mag deficiency that identical feeding wouldn't cause on harder water.
Your EC baseline is part of the maths
Say your tap water reads 0.4 EC and you're targeting a feed EC of 1.2. That's 0.4 from the tap plus 0.8 from your nutrients not the full 1.2. Your nutrients are only doing part of the work. This matters most for seedlings and light feeders, where the target EC is low enough that tap water alone can make up a meaningful share of it. As a rule of thumb, always subtract your source water's EC from your target EC before working out how much nutrient to add.
Chlorine and chloramine aren't the same problem
Most municipal supplies use one or the other to keep water safe to drink.
Chlorine will dissipate on its own if you leave water to sit overnight with the lid off.
Chloramine is more stable and won't dissipate the same way it needs activated carbon filtration or a dechlorinating additive to remove. If you're running beneficial bacteria or mycorrhizal inoculants, chloramine can kill them off before they get a chance to establish.
RO water: the clean-slate option
Reverse osmosis strips almost everything out, leaving near-pure H₂O at 0.0–0.05 EC. That's not automatically an advantage it just shifts the responsibility onto you.
Zero alkalinity means zero resistance to pH change, so a small amount of acid or base swings RO water's pH hard and fast. Ironically, it's harder to hold stable than water with some mineral content in it.
The big one for coco growers is missing calcium and magnesium. Coco coir binds Ca and Mg onto its exchange sites and releases Na/K back in return, so if your water isn't replacing what gets locked up, you end up with textbook Cal-Mag deficiency pale new growth, interveinal chlorosis, weak stems, even on a nutrient line that technically contains some.
There's no mineral floor either. Tap or well water usually supplies incidental trace minerals and some buffering that the soil's microbial ecosystem relies on. Straight RO takes that away, so soil growers can see mineral drift or a drop in microbial activity over time if they're not remineralising.
The upside is total control you're building your nutrient solution from a true zero, with no unknowns from the tap.
The trade-off is Cal-Mag every run, and a pH that needs closer watching with no mineral buffer to lean on. RO tends to be the preferred choice for precision growers and hydro systems, where consistency matters more than convenience.
Match your nutrients to your water
Some nutrient lines are formulated specifically for hard or soft water. Run a hard-water formulation on water that's already hard and you can push calcium and magnesium higher than you meant to. If your manufacturer offers both versions, check which one fits your supply it's not a marketing distinction, it's chemistry.
A simple starting protocol
- Test your tap water for EC and pH before anything else. We can run this for you in-store, or a decent meter like the Bluelab pH & EC Starter Pack will pay for itself in the first grow.
- EC above 0.5? Factor it into your feed calculations rather than ignoring it.
- pH won't hold still? Consider filtering your source water.
- Cal-Mag symptoms on soft water? Add a dedicated supplement rather than pushing your base nutrient harder.
- One test gives you a much clearer picture of everything that follows.
Your water is an input, same as your nutrients. Know what's in it before you start adjusting around it most water-related grow problems have simple solutions once you actually know what you're working with, and that starts with a single test, not another bottle of additive.