If your tea has a film on top, your water is the reason. It isn’t the tea, the mug or the milk.
The film forms when two things in hard water, calcium and bicarbonate, react with the tea as it cools. Take either away and the film mostly disappears. Go too far the other way, though, and black tea tastes thin and harsh. The best cup sits in the middle: soft, but not empty.
If you’re not sure, buy this
For black tea: Tesco British Natural Mineral Water (5 L bottle) or Aqua Pura, or Waitrose Essential Still at Waitrose. These are the same waters we recommend for coffee, and for a related reason: low bicarbonate. They make a clear cup with enough body to stand up to milk.
For green tea: Sainsbury’s Caledonian. Green tea wants softer water than black tea, and Caledonian is the softest everyday option we’ve found that still has some minerals in it.
UK bottled waters for tea, compared
| Bottled water | Where to buy | Typical price¹ | Total hardness (mg/L) | Bicarbonate (mg/L) | Black tea | Green tea |
|---|---|---|---|---|---|---|
| Tesco British Natural Mineral Water, 5 L | Tesco | £1.50 / 5 L | 63 | 46 | Good | Good |
| Waitrose Essential Still | Waitrose | £0.25–0.35 | 63 | 46 | Good | Good |
| Aqua Pura | Tesco, Sainsbury’s, Asda, Morrisons | £0.25–0.45 | 76 | 55 | Good | Good |
| Sainsbury’s Caledonian | Sainsbury’s | £0.20–0.30 | 63 | 32 | Good | Best |
| Volvic | All major supermarkets | £0.50–0.80 | 62 | 71 | Good | Good |
| Deeside | Tesco, Waitrose | £0.80–1.50 | 22 | 27 | Thin | Good |
| Highland Spring | All major supermarkets | £0.42–0.67 | 143 | 150 | Some film | Muted |
| Tesco British Natural Mineral Water, 2 L | Tesco | £0.80 | 137 | 140 | Some film | Muted |
| Asda Still | Asda | £0.78 | 158 | 166 | Some film | Muted |
| Buxton | All major supermarkets | £0.33–0.50 | 216 | 248 | Film | Muted |
| Evian | All major supermarkets | £0.67–1.00 | 307 | 360 | Heavy film | Muted |
¹ Per bottle, as last checked (most in March 2026). Bottle sizes vary, so compare per litre on the shelf.
Why green tea is fussier
Green tea is brewed cooler and has a lighter flavour, so there’s less to cover up what the water is doing. Even moderate hardness can make it taste dull. Japanese green teas are the most sensitive: a total hardness under about 40 mg/L suits them best.
Your tap water might already be fine
Much of Scotland, Wales and the North West has soft tap water, and if that’s you, a bottle won’t improve your tea by much. In London and the South East, tap water is usually above 200 mg/L total hardness, and the film is hard to avoid without changing water.
Common questions
Why does my tea have scum on top?
Your water is hard. Calcium and bicarbonate react with compounds in the tea and form a film at the surface. Softer water, or a lower-bicarbonate bottled water, removes most of it.
What's the best bottled water for tea in the UK?
For black tea, Tesco British Natural Mineral Water (5 L bottle), Aqua Pura or Waitrose Essential Still. For green tea, Sainsbury’s Caledonian.
Does a water filter help with tea?
Yes, more than it does for coffee. A jug filter removes some calcium and bicarbonate, which is enough to reduce the film noticeably.
Is distilled water good for tea?
Not for black tea. It brews a thin, harsh cup. It’s closer to workable for delicate green teas, but a soft bottled water is better.