Ask ten hobbyists what the perfect aquarium pH is and nine will say 7.0. The tenth knows it is the wrong question. In Part 1, we looked at all seven parameters. Now we take the most misunderstood one.
What pH Actually Measures
pH measures how acidic or alkaline your water is on a scale of 0 to 14, where 7 is neutral. The scale is logarithmic. Water at pH 6 is ten times more acidic than water at pH 7, and a hundred times more acidic than pH 8. This is why a fast swing, even a small-looking one, is hard on fish.
Safe pH Ranges for Popular Fish
|
Fish group |
Comfortable pH range |
|---|---|
|
Guppies, mollies, platies |
7.0 to 8.0 |
|
Tetras and rasboras |
6.0 to 7.5 |
|
Bettas |
6.5 to 7.5 |
|
Goldfish |
7.0 to 8.0 |
|
Neocaridina shrimp |
6.5 to 7.5 |
These are working ranges, not hard limits. Always check the needs of your specific species.
Why pH Keeps Changing
- Nitrification uses up KH. As bacteria process waste, they consume the water's buffering capacity, so the pH slowly drifts down.
- Fish and plants release CO2. At night, plants release CO2 too, so pH often dips before lights come on.
- CO2 injection lowers pH on purpose. If your buffer is weak, it can drop too far. See our CO2 series.
- Hardscape and decor. Driftwood and almond leaves lower pH. Limestone, some rocks and shells raise it.
- Water changes with water of a different pH than the tank.
- Overstocking and overfeeding pile on waste, which pushes pH down faster.
The common thread is KH, the buffer that holds pH steady. We cover that in Part 3.
The Chemical Shortcut Trap
Powders and drops that promise instant pH correction give a quick change that usually drifts back within days. Dosing repeatedly creates a yo-yo effect that stresses fish more than the original number ever did. A better route is to keep KH stable, do regular water changes, and choose fish that suit your tap water.
How to Adjust pH Gently
- Pick fish that match your water first. It is the easiest fix there is.
- To lower pH slowly: add driftwood or Indian almond leaves.
- To raise pH slowly in soft water: use crushed coral or limestone in the filter.
- To support pH and KH balance naturally: consider our natural granular product, Indica.
- Change water in small steps: 20 to 30 per cent at a time, dechlorinated and temperature-matched.
- Never push pH by more than about 0.3 to 0.5 a day.
When to Worry
Watch for gasping at the surface, lethargy, clamped fins or fish hiding after a water change. If you see these, test pH and KH straight away, do a partial water change with matched water, and avoid sudden chemical corrections. For help with new arrivals, see our guide on how to acclimate new fish.
👉 Shop TCWS Water Test Strips | Shop Sunken Garden Anti-Chlorine | Next: Part 3, GH, KH and TDS
