By Alexis Rohlin • Updated Mar 24, 2022
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Fish and other aquatic life depend on a precise pH range for optimal health. When the pH drifts too low (acidic) or too high (alkaline), it can cause stress, disease, and even death.
pH measures a solution’s acidity or alkalinity by quantifying its hydrogen ion concentration. A lower pH indicates more hydrogen ions and greater acidity; a higher pH indicates fewer hydrogen ions and a more basic environment. Pure water is neutral at pH 7. Everyday examples illustrate the scale: milk (pH 6), lemon juice (pH 2), battery acid (pH 1), baking soda (pH 9), and lye (pH 14).
Acidic or basic substances can alter water chemistry by shifting pH. In an aquarium, added or removed fish, certain rocks, and chemical additives can raise or lower pH, directly influencing the form of ammonia present. When pH drops, ammonia (NH₃) converts to the less toxic ammonium (NH₄⁺), while a rise in pH shifts the balance toward the toxic free‑ammonia form, which can rapidly kill fish.
Each species has a preferred pH range that mirrors its natural habitat. For instance, koi thrive around pH 7.5–8.2, Oscars favor a slightly acidic range of pH 6.5–7.0, and many African cichlids require a more alkaline setting near pH 8.5. Maintaining a neutral pH of 7 is a good baseline for many common aquarium fish.
Water with a pH below 5 is excessively acidic and can burn fish skin, impair eggs, and inhibit hatching. Conversely, a pH above 8 can cause chemical burns and “chapping” of fish scales. Juvenile fish are particularly vulnerable to sharp pH changes, so stability is key.
Regularly test your tank with a reliable pH kit. To raise a low pH, add buffering agents such as limestone or crushed coral, aerate to remove excess CO₂, or use an alkaline buffer. To lower a high pH, consider acid buffers, a water softener, or filtering through peat moss. Always adjust gradually—no more than 0.2–0.3 pH units per day—to give fish time to acclimate.
