The intuition, and why it runs backwards
If tap water contains things that are not tea, removing them ought to leave the tea more clearly itself. That reasoning is why distilled water gets tried, and it is why the result is such a surprise: the cup comes out thin, hollow and oddly sharp, with the aroma present but somehow unsupported. Nothing has been contaminated and nothing has been lost from the leaf. What has happened is that a variable most people did not know was doing work has been set to zero. Water in tea is not a background against which flavour appears; it is a participant. Its dissolved content changes what comes out of the leaf, how the extracted material behaves once it is in solution, and how the drink feels in the mouth. Removing every dissolved ion does not neutralise those effects — it removes the ones that were helping along with the ones that were not. The useful comparison is with cooking rather than with cleaning. Unsalted stock is not a purer stock; it is a differently constructed one, and no amount of reducing it produces what the salt would have. The same relationship holds here, and it is why the first instinct on tasting distilled-water tea — to use more leaf — makes the problem worse rather than better.
What the ions were doing
Calcium and magnesium are the two that hardness measures, and they take part in extraction rather than merely accompanying it. Both associate with some of the polyphenols coming off the leaf, which changes how much of that fraction stays in solution and how it reads on the palate. Some of the perceived body of tea brewed in moderately mineralised water comes from that interaction rather than from more material having been extracted, which is why a hardness figure predicts mouthfeel better than it predicts strength. Water with essentially none of either extracts a subtly different profile, and the loss shows up most in the middle of the palate — the part between the first impression and the finish. That is why the fault is described as hollowness rather than weakness, and why people reaching for the word usually settle on an architectural metaphor rather than a quantitative one. It is worth being clear about the strength of this claim. The direction is well supported and reproducible in side-by-side tasting by people who do not know which cup is which. The precise mechanism, and any statement about which compound binds what and in what proportion, is an area where popular tea writing is considerably more confident than the published work justifies, and TeaHQ declines to repeat the specifics.
Buffering, which is the part that matters most
Tea liquor is mildly acidic, and the acidity develops as brewing proceeds rather than being present from the start. Water that carries bicarbonate resists that shift — that is what buffering is — and the brew settles somewhere stable and stays there while it is drunk. Water with no dissolved carbonate has no such resistance, so the pH of the brew is set almost entirely by the tea itself and moves as the steep runs and again as the cup cools. The result is a liquor whose balance drifts while you are drinking it, and whose acidity often reads as a thin brightness at the front of the palate with nothing behind it. This is the single largest contributor to the flatness people describe, and it explains why the fix is not more leaf: adding leaf to unbuffered water adds acid along with everything else, giving a more concentrated version of the same hollow shape. It also explains the mirror-image fault at the other extreme. Heavily carbonated water buffers so strongly that the brew never reaches the acidity it should, which dulls aroma and darkens the liquor. Both ends of the range fail, and they fail differently, which is why a single number for water quality is not much use.
Water has a taste of its own, and pure water has none
Before any tea is involved, mineralised water and distilled water do not taste the same. Distilled water is widely described as flat, faintly metallic or simply absent, and that absence is carried straight into the cup — the drink is over ninety-nine per cent water, so whatever the water is like in the glass is most of what the tea is like in the glass. This is a separate argument from the extraction one and it is routinely underestimated, because people testing water for tea taste the tea rather than the water, and by then the two effects are inseparable. Tasting the two waters cold, side by side, before brewing anything with either, is the quickest way to understand where a good deal of the difference comes from, and it takes a minute. It also recalibrates the question. Someone who finds that they can taste a clear difference between two plain waters should expect that difference to survive into the tea; someone who genuinely cannot should be sceptical of elaborate water advice, since the effect they are being asked to chase is smaller than their own detection threshold. Neither outcome is a failing, and knowing which one applies saves a great deal of money.
Distillation, deionisation and reverse osmosis are not one thing
They are often used interchangeably in tea writing and they produce different water. Distillation boils and condenses, leaving dissolved solids behind in the boiler and driving off dissolved gases along the way, so distilled water is both mineral-free and degassed. Deionisation exchanges ions on a resin, removing charged species while leaving uncharged organic material largely untouched — laboratory-grade in one specific respect and not in others. Reverse osmosis pushes water through a membrane that rejects most dissolved content but not all of it, so RO water typically retains a small and variable residue rather than approaching nothing, and the residue depends on the membrane, its age and the pressure behind it. In practice RO is the one most likely to be met at home, because domestic under-sink systems and many countertop filters produce it, and it is the least extreme of the three. That is why RO tea is usually described as thin rather than as strange, while distilled tea provokes stronger reactions. It also means that the advice you read about one does not transfer cleanly to another, and that a reader diagnosing their own flat tea should find out which of the three their equipment actually makes before concluding anything.
Where pure water is a starting point rather than a mistake
The enthusiast practice of building brewing water starts from RO or distilled precisely because it is a blank. A known quantity of specific salts is added back to a known volume, and the result is repeatable in a way tap water can never be, because tap water is a blend that changes with the season and the source. That is a legitimate use of pure water and a genuinely small niche, and it is worth separating from the ordinary case rather than treating the two as points on one scale. In the ordinary case the choice is between filtered tap water and pure water with nothing added, and filtered tap water wins comfortably almost everywhere. There is a second legitimate use: regions where the supply is so heavily mineralised that no ordinary filter brings it into range, where near-total removal followed by a modest addition is the only practical route. Both of these are real. Neither is a reason for anyone whose tap water is merely chlorinated to buy a distiller, and the guide to filters covers the cheaper answer that solves that problem directly.
Where the claim is over-stated
Pure water is not equally bad for every tea, and the blanket statement that it ruins tea is stronger than the evidence supports. The teas that suffer most are the ones whose appeal is body, sweetness and a long finish — shaded Japanese green tea, good white tea, aged dark tea — because those depend on precisely the fraction and the mouthfeel most affected. Robust, heavily roasted and strongly flavoured material fares considerably better, since what it offers is aroma and intensity rather than texture. A smoked black tea or a heavily roasted oolong made with distilled water is recognisably itself, and someone comparing the two would call the difference small. Cold brewing is another partial exception, because the low temperature already narrows what is extracted and the water’s contribution is proportionally smaller. TeaHQ states the direction confidently and declines to state that any particular tea is undrinkable made this way, both because that has not been established and because it is unusually easy to check for yourself: two identical brews, one water changed, is a fifteen-minute experiment that settles the question for your own palate better than any general claim can.