The UK has dramatically different tap water across regions — from ultra-soft Scottish highland water to the very hard chalk-filtered water of East Anglia. Understanding your local supply is the single most important starting point for setting up any freshwater aquarium. The wrong starting water means constant battles with pH, fish stress and failed breeding attempts.
⚠️ Seasonal variation: UK tap water hardness can fluctuate ±2–4°dH between summer and winter as water suppliers blend from different sources. The ranges below reflect typical annual averages. Always test your own tap water before setup — we recommend the JBL GH/KH test kit for accurate results.
Data sourced from water supplier annual quality reports and verified with in-field test kits. Values are typical ranges; test your own supply before use.
| Region / Supplier | pH | GH (°dH) | KH (°dH) | Type | Best For | Needs Softening For |
|---|---|---|---|---|---|---|
| London (Thames Water) | 7.8–8.0 | 18–24 | 12–16 | Very Hard | African cichlids, livebearers | Discus, tetras, most soft-water species |
| East Anglia / Cambridge (Anglian Water) | 7.6–8.2 | 20–28 | 14–20 | Very Hard | African cichlids only | Almost all species except hard-water cichlids |
| Birmingham (Severn Trent) | 7.2–7.8 | 12–18 | 8–14 | Hard | African cichlids, community | Discus, SA cichlids (50–60% RO) |
| Bristol (Bristol Water) | 7.4–7.8 | 14–20 | 10–14 | Hard | African cichlids, community | Discus (60–70% RO) |
| Cardiff (Dŵr Cymru) | 7.2–7.6 | 10–16 | 7–12 | Moderate–Hard | Community, cichlids | Discus (40–60% RO) |
| Leeds / Bradford (Yorkshire Water) | 7.0–7.6 | 8–14 | 6–10 | Moderate | Planted, community | Discus (30–50% RO) |
| Sheffield (Yorkshire Water) | 7.0–7.6 | 8–14 | 6–10 | Moderate | Planted, community | Discus (30–50% RO) |
| Manchester (United Utilities) | 7.0–7.6 | 6–12 | 5–9 | Moderate–Soft | Community, planted | Discus (20–40% RO); African cichlids (add minerals) |
| Liverpool (United Utilities) | 7.0–7.4 | 5–10 | 4–8 | Soft–Moderate | Community, planted | African cichlids (add minerals) |
| Newcastle (Northumbrian Water) | 7.0–7.4 | 5–10 | 4–8 | Soft–Moderate | Community, planted | African cichlids (add minerals) |
| Belfast (NI Water) | 7.0–7.4 | 5–12 | 4–9 | Soft–Moderate | Community, planted | African cichlids (add minerals) |
| Exeter / Devon (South West Water) | 7.0–7.4 | 4–10 | 3–7 | Soft | Planted, discus, SA cichlids | African cichlids (add minerals) |
| Edinburgh (Scottish Water) | 6.8–7.4 | 3–8 | 2–6 | Soft | Discus, tetras, soft-water SA species | African cichlids (add significant minerals) |
| Glasgow (Scottish Water) | 6.8–7.2 | 2–6 | 1–4 | Very Soft | Blackwater, discus, wild-type tetras, SA cichlids | African cichlids (add significant minerals) |
| Inverness / Highlands (Scottish Water) | 6.6–7.2 | 1–4 | 1–3 | Very Soft | Blackwater, wild discus, apistogramma | African cichlids; may need KH buffer even for community fish |
°dH = degrees of hardness. Multiply by 17.9 to convert to ppm.
What These Numbers Mean for Your Fish
Your local water type directly determines which species will thrive without ongoing water treatment, and which require consistent adjustment:
| Water Type | GH | Ideal Species — No Treatment | Species Needing Adjustment |
|---|---|---|---|
| Very Soft (Scotland, SW England) | 1–6°dH | Discus, wild tetras, apistogramma, blackwater species | African cichlids, livebearers, goldfish (add minerals) |
| Soft–Moderate (NW England, NI) | 5–12°dH | Community tetras, corydoras, planted tanks, most barbs | African cichlids (add minerals); discus (partial RO) |
| Moderate–Hard (Midlands, Wales, Yorkshire) | 10–18°dH | General community, livebearers, goldfish | Discus (50–70% RO); SA cichlids (30–50% RO) |
| Hard–Very Hard (London, East Anglia) | 18–28°dH | African cichlids, Rift Lake species, livebearers | All soft-water species (70–80% RO minimum) |
Reverse osmosis (RO) filters strip almost all minerals from tap water, producing near-pure water (GH <1°dH, KH <1°dH). You then blend RO water with tap water to reach your target parameters.
RO Unit Output (typical domestic membranes)
| Membrane Size | Output per Hour | Suitable For |
|---|---|---|
| 50 GPD (gallons per day) | ~7.5 litres/hr | Tanks up to 150L (frequent top-ups required) |
| 100 GPD | ~15 litres/hr | Tanks 100–300L; most home setups |
| 200 GPD | ~30 litres/hr | Tanks 300L+; commercial systems; discus breeding |
RO:Tap Mixing Ratios — Target GH 4–8°dH (planted tanks, discus)
| Region | Tap GH (°dH) | RO Ratio Needed | Result GH (approx.) |
|---|---|---|---|
| London / East Anglia | 20–25 | 70–80% RO / 20–30% tap | 4–8°dH |
| Birmingham / Bristol | 14–18 | 55–65% RO / 35–45% tap | 5–8°dH |
| Yorkshire / Midlands | 10–14 | 40–55% RO / 45–60% tap | 5–8°dH |
| Manchester / Liverpool | 6–10 | 20–40% RO / 60–80% tap | 4–8°dH |
| Scotland | 2–6 | 0–20% RO / 80–100% tap | 2–6°dH (often no RO needed) |
Always re-test after blending. Add a GH/KH buffer (e.g. Seachem Equilibrium) to RO water before use to restore essential minerals.
If your tap water is soft (Scotland, NW England, SW England) and you want to keep African cichlids (target: GH 10–20°dH, KH 10–15°dH, pH 7.8–8.5), you need to add mineral salts.
| Product | Dose (per 40 litres) | Effect on GH | Effect on KH | Effect on pH |
|---|---|---|---|---|
| Seachem Malawi Buffer | 1 tsp (5g) | +2–3°dH | +2–3°dH | Raises to 7.8–8.2 |
| Seachem Tanganyika Buffer | 1 tsp (5g) | +2–3°dH | +3–4°dH | Raises to 8.0–8.5 |
| Rift Lake Cichlid Salts (API) | 2.5ml per 40L | +3–5°dH | +2–4°dH | Raises slightly |
| Limestone/Tufa Rock in tank | Passive leaching | +2–8°dH (variable) | +2–6°dH (variable) | Gradually raises to 7.8–8.5 |
Adjust gradually — rapid pH changes stress fish. Raise by no more than 0.2 pH units per day.
Most UK water suppliers use chloramine (a chlorine–ammonia compound) rather than plain chlorine as a disinfectant. This matters because:
- Chlorine — evaporates from standing water within 24 hours
- Chloramine — does NOT evaporate. Requires a dechlorinator that explicitly states it neutralises chloramine (e.g. Seachem Prime, Tetra AquaSafe)
- Chloramine + RO: RO membranes are damaged by chloramine over time — fit a carbon pre-filter before your RO membrane
| Supplier | Disinfectant | Dechlorinator Required |
|---|---|---|
| Thames Water (London) | Chloramine | Yes — chloramine-specific |
| Anglian Water (East Anglia) | Chloramine | Yes — chloramine-specific |
| Severn Trent (Midlands) | Chloramine | Yes — chloramine-specific |
| Yorkshire Water | Chlorine (some areas chloramine) | Yes — use chloramine-safe product regardless |
| United Utilities (NW England) | Chlorine primarily | Yes — standard dechlorinator sufficient |
| Scottish Water | Chlorine primarily | Yes — standard dechlorinator sufficient |
When in doubt, always use a dechlorinator rated for chloramine. Seachem Prime and Tetra AquaSafe Plus both neutralise chloramine and are widely available in the UK.
How to Test Your Own Tap Water
The regional averages above are a guide — your actual supply can vary by postcode, season and network. Always test before setup:
- JBL GH/KH Test Kit — reliable liquid drop test for both parameters, available from most UK aquatic retailers (~£8–12)
- API pH Test Kit — more accurate than strips for pH; covers the full 6.0–7.6 range
- JBL PH 7.4–9.0 Test — for hard-water / African cichlid setups needing the higher pH range
- TDS meter — measures total dissolved solids (ppm); useful for verifying RO water purity (target <20 ppm for discus setups)
Not sure what your water needs?
All our maintenance plans include full GH, KH and pH testing on every visit. Our health check service includes a full water analysis and written report. Or contact Adnan or Hasan — we'll tell you exactly what your local supply needs for your chosen species.