pH in the Aquarium

pH is a measure of the concentration of hydrogen ions in water, expressed on a logarithmic scale from 0 (very acidic) to 14 (very alkaline), with 7 neutral. Because the scale is logarithmic, each whole number represents a tenfold change, which is why even modest-looking pH shifts can be significant to fish.

What fish prefer. Species evolved for their native waters: soft-water Amazonian fish such as tetras and angelfish favour a slightly acidic pH (around 6.0–7.0), while livebearers like guppies and many African species prefer alkaline water (around 7.5–8.0). Most tap water sits somewhere in the neutral range, and a great many hardy community fish adapt comfortably to it.

Stability over perfection. For the typical hobbyist, keeping pH stable is usually more important than forcing it to an ideal number. Fish tolerate a steady value slightly outside their preferred range far better than they tolerate rapid swings. Chasing an exact pH with chemical adjusters often backfires, causing the very fluctuations that harm fish. pH is also linked to carbonate hardness (KH), which buffers the water and resists pH change — low KH lets pH swing easily.

Practical approach. Test your tap water's pH and hardness first, then choose fish that suit it rather than fighting your water. Where adjustment is genuinely needed, natural methods — driftwood and peat to lower pH, or crushed coral to raise and buffer it — are gentler and more stable than sudden chemical dosing. In short: know your number, keep it steady, and stock to match.