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pH is one of the first things new fishkeepers test, and one of the most common reasons people reach for a bottle of chemicals. But pH doesn't act alone. It's controlled by your water's hardness, and in particular its KH. Understanding the three together is the key to stable water and healthy fish.
The three numbers, in plain English
| Measure | What it is | Why it matters |
|---|---|---|
| pH | How acidic or alkaline the water is, from 0 to 14. 7 is neutral. | Affects fish health, breeding and how toxic ammonia is. |
| GH (general hardness) | Dissolved calcium and magnesium. | Fish and shrimp need these minerals for bones, scales and shells. |
| KH (carbonate hardness) | Carbonates and bicarbonates that buffer the water. | The higher the KH, the more stable the pH. |
Think of KH as a sponge that soaks up acid. When KH is high, your pH stays put. When KH is very low, pH can drop suddenly, sometimes overnight, in what's known as a pH crash.
Ideal ranges for common fish
| Fish | pH | Hardness |
|---|---|---|
| Most community fish (tetras, rasboras, corydoras) | 6.5–7.5 | Soft to moderate |
| Discus, cardinal tetras, apistogramma | 5.5–6.8 | Soft |
| Livebearers (guppies, mollies, platies) | 7.0–8.0 | Moderate to hard |
| African rift lake cichlids | 7.8–8.6 | Hard |
| Goldfish | 7.0–8.0 | Moderate |
| Caridina shrimp (crystal, bee) | 5.8–6.8 | Very soft |
Most tank-bred fish adapt to a wider range than these figures suggest. Stability matters far more than chasing an exact number. A steady pH of 7.6 is better for a tetra than a pH that swings between 6.5 and 7.2.
How to test
Test pH, KH and GH from the tank and from your tap. Many UK regions, especially the South East, have hard, alkaline tap water with a pH around 7.5–8.0; parts of Scotland, Wales and the North have much softer water.
- The API Freshwater Master Test Kit covers pH alongside ammonia, nitrite and nitrate.
- For fast, repeatable readings, a digital tester such as the Hanna pHep Pocket pH Tester is ideal. Calibrate it with pH 7.01 buffer sachets.
- Choose a kit with the right range: the NT Labs Broad pH kit is a good all-rounder, while the API High Range pH kit suits hard-water and cichlid tanks.
Browse all aquarium test kits.
Why pH drifts
- pH falling: waste builds up and produces acids, gradually using up the KH. This is very common in tanks with infrequent water changes. Driftwood, peat and catappa leaves lower pH too. Injected CO2 lowers pH during the day.
- pH rising: limestone, some rocks, coral sand and shells dissolve and add hardness. Hard tap water raises pH with every water change.
How to lower pH safely
- Use natural materials. Driftwood such as Mopani Wood or Aqua One Driftwood and botanicals release tannins that gently lower pH and give a natural blackwater look. See our aquascaping wood and botanicals.
- Soften the water with RO. This is the most reliable method. Mixing tap water with reverse osmosis water lowers KH and GH, so pH settles lower and stays there. A unit such as the Glamorca RO Unit produces RO water at home. Read our guide to RO water.
- Use a buffer, not a quick fix. If you need chemical help, a buffer such as Seachem Acid Buffer lets you set a target. Quick "pH down" products such as API pH Down work, but in hard water the pH will drift back as KH pushes it up again. Use them carefully and in small steps.
- Use an active soil substrate for soft-water fish and shrimp. Planted substrates buffer the water to slightly acidic. See our planted substrates.
How to raise pH safely
- Do more water changes. If pH has fallen because KH has been used up, regular water changes are often the whole answer.
- Add buffering. Seachem Alkaline Buffer raises KH and pH together, so the change sticks. API pH Up and Fritz pH Higher are quicker adjusters for small corrections.
- Add GH for soft water. If you use RO water, remineralise it. Seachem Equilibrium adds the calcium, magnesium and potassium fish and plants need.
- Use hardscape for hard-water fish. For African cichlids, limestone-type rock or a coral sand substrate holds pH high naturally.
Golden rules
- Change pH by no more than 0.2–0.3 units per day.
- Never adjust pH with fish in a bucket or bag; adjust the tank slowly instead.
- Fix the KH, not just the pH. If KH is below 3°dKH, raise it before anything else.
- Prepare new water to match the tank before every water change.
- Choose fish that suit your tap water. It's much easier than fighting it.
For more on keeping your water stable, read How to Use Water Conditioners and Test Kits the Right Way, or browse our Seachem freshwater conditioners.
