Why the water is not a neutral background
Brewing is dissolution, and a solvent that already contains dissolved ions behaves differently from one that does not. That shows up in two separate ways. First, extraction itself changes: what the water pulls out of the leaf, and how readily, depends partly on its mineral content. Second, and more visibly, the extracted compounds go on reacting in the cup — some of them bind with dissolved metals and change colour, and some fall out of solution altogether. Anyone who has moved house and found that a familiar tea no longer tastes familiar has met this. The tea did not change.
Hardness: calcium and magnesium
Hardness is a measure of dissolved calcium and magnesium. A moderate amount is generally regarded as helpful — completely soft or distilled water tends to give thin, hollow tea, because there is very little dissolved to work with — and a large amount flattens aroma and dulls flavour. The two ions are not equivalent in their effects, and speciality coffee has argued at length about the difference; for tea the practical point is coarser. If your kettle scales quickly, your water is hard, and your tea is probably duller than the same leaf would be elsewhere. That is a real effect and it is not the whole story.
Alkalinity is the variable that does most of the damage
Alkalinity — chiefly dissolved bicarbonate — measures the water’s capacity to resist a change in pH, and it travels with hardness often enough that the two get conflated. They are different things and alkalinity is the one that shows in the cup. Tea liquor is mildly acidic, and highly alkaline water buffers that acidity away, shifting the liquor towards neutral. The consequences are visible before they are tasted: the colour deepens and dulls towards brown, the brightness goes, and the drink flattens. This is the mechanism behind the household trick of adding a squeeze of lemon to iced tea and watching it lighten several shades, and behind the pinch of bicarbonate of soda that some sweet tea recipes use deliberately in the opposite direction.
Chlorine and chloramine
Most treated tap water carries a disinfectant residual, and it is the most immediately detectable water fault in tea because it is an aroma problem rather than a subtle extraction one. Free chlorine is volatile: standing water uncovered for an hour, or boiling it, removes a good deal of it. Chloramine, which many utilities now use instead because it persists longer in the pipes, does not leave nearly so readily and is not driven off by boiling to any useful degree. Activated carbon removes both. If a filter jug improves your tea noticeably and immediately, this is usually why.
What a TDS number does and does not tell you
A total dissolved solids meter measures electrical conductivity and converts it, using an assumed factor, into an estimate of dissolved material. It is a single number standing in for a composition, and two waters reading identically can brew very differently — one high in calcium and low in bicarbonate, the other the reverse. The number is useful for spotting that two sources differ, and close to useless for predicting how. A water utility’s published report, which separates hardness from alkalinity, is more informative than any pen. Treating a TDS figure as a target to hit is the point at which water optimisation stops being useful.
The film on the surface
TeaHQ's own reading of well-attested general knowledge, with no single source behind it.
The iridescent film that forms on a cup of black tea left standing is not dirt and not a fault in the leaf. It is generally attributed to tea compounds combining with calcium from the water at the surface, where oxygen is available — which is why it appears readily in hard-water areas and barely at all in soft ones, why it is worse in an open cup than a covered one, and why acidifying the tea with lemon disperses it. It is harmless and cosmetic, and it is a useful visible indicator of what your water is doing to the drink.
What to change, in order
Filter, if your water is chlorinated or hard: it is the cheapest intervention and usually the largest. Draw water fresh and boil it once, since repeatedly boiled water loses dissolved gases and tastes flatter. Beyond that, resist the recipes. Remineralising distilled water to a formula is a real practice in a small enthusiast corner, and it delivers a small refinement on top of decisions — temperature, ratio, and giving the leaf room — that are worth far more and cost nothing. TeaHQ’s position is that water is worth getting reasonable and not worth engineering.