pH

For most freshwater community tanks the ideal pH is 6.0–7.5, and almost any steady value in that band beats a "perfect" number that swings. Below 5.0 or above 9.0 livestock is in acute danger. Because the scale is logarithmic, a one-unit drop means ten times the acidity — an overnight swing does more harm than a wrong-but-stable reading ever will.

What is pH?

pH is the negative log of the hydrogen ion concentration. The scale is logarithmic — pH 6 has ten times the H⁺ concentration of pH 7, a hundred times pH 8. Most freshwater aquariums sit between 6.0 and 8.0.

The tank pushes it around daily from both sides: respired CO₂ and nitrification pull down, photosynthesis and surface off-gassing push up. How far each push actually moves the needle depends on KH — pH and carbonate hardness are two halves of the same system, which is why every pH problem deserves a KH test first.

The ideal pH level in a freshwater aquarium

For a general community tank the ideal pH is 6.0–7.5, and most tank-bred fish settle anywhere in that band. But pH is the one parameter where steadiness beats the target: a stable 7.8 is far better than a 6.8 that swings half a point every night. Match the band to the livestock — soft-water species toward the bottom, rift-lake cichlids above it — then hold it there.

Why a steady pH matters

A steady pH is far more important than a "correct" one. Most fish in the hobby have been tank-bred for generations and tolerate a wide pH range as long as it doesn't swing. The exceptions: dedicated soft-water breeding tanks (discus, wild Apistogramma), African rift-lake cichlid tanks (need pH ≥ 8), and tanks where pH drops sharply at night because KH is too low to buffer the CO₂ cycle.

For planted tanks running CO₂ injection, pH is the live readout of how much CO₂ is dissolved. A 1.0 pH drop from non-injected to lights-on usually means CO₂ is in the target band.

There's a second, sneakier reason to watch pH: it decides how toxic your ammonium is. NH₄⁺ and NH₃ are the same nitrogen in two forms, and pH sets the split — at 25 °C and pH 7.0 only about 0.6 % of total ammonia is the toxic NH₃ form; at pH 8.0 it's around 5 %; at 8.5 nearly 15 %. The same test-kit reading is roughly twenty-five times more dangerous one and a half pH units higher (pH 7.0 → 8.5). This is also the mechanism behind the classic "the water change killed my fish" story: in an acidic tank with a stalled cycle, ammonium quietly accumulates in its harmless form — until a big change lifts the pH and converts it to ammonia in an afternoon.

How to test pH

A calibrated pH meter is worth the cost if you run CO₂ or breed soft-water species. Otherwise a liquid drop test (JBL pH, Tetra pH) or a strip is fine for monthly checks. Test at the same time of day every time — pH cycles by 0.5–1.0 over a CO₂-injected day.

Two pitfalls catch almost everyone. Fresh tap water is often supersaturated with CO₂ from the pressurised line, so it reads artificially low at the faucet — let a glass stand (or run an airstone in it) for 24 hours and the pH can climb a full point; that degassed reading is the one your tank will actually settle at. And meters drift: calibrate monthly against pH 7.00 and 4.01 buffers, store the probe in storage solution rather than RO water, and be suspicious of any meter that never changes its mind.

Signs pH is too high — or too low

Below 5.5: most fish stressed; acid burn on gill membranes within days. Between 5.5 and 6.5: planted-tank territory, fine for South American and Southeast Asian species. Between 6.5 and 7.5: middle ground, suits most community fish. Above 8.0: alkaline territory, suits African cichlids and brackish setups but stresses soft-water species.

The zones are about matching species; the acute symptoms are about speed. A crash shows up as gasping at first light, excess slime coat, reddened gills and frantic darting — and below roughly pH 6 the nitrifying bacteria slow down, so a long-stable tank can start showing ammonium again. A climb is quieter: soft-water fish wash out in colour, and a high pH riding on a high KH pushes iron and other trace metals into insoluble forms, so new leaves come in yellow no matter how faithfully you dose.

How to lower pH

CO₂ injection lowers pH naturally and reversibly — the cleanest lever for a planted tank. For non-planted tanks, peat in the filter or dilution with RO water brings it down gently. Chasing pH down with acids is a trap: without KH to buffer, it bounces straight back and swings the fish around in the process.

The CO₂ route is also the measurable one. Dissolved CO₂ follows

CO₂ (mg/L) = 12.839 · KH (°dKH) · 10^(6.35 − pH)

so at KH 4, taking the pH from 7.4 down to 6.8 moves the tank from ~5 to ~18 mg/L of CO₂ — inside the planted-tank band. The CO₂ without a meter guide turns that formula into a lookup table you can read at the tank.

How to raise pH

Check the cheap explanation first: accumulated CO₂. A tank with weak surface agitation can carry enough respired CO₂ to sit half a point below its natural pH, and an airstone run overnight tells you whether you have a chemistry problem or a circulation one. If the buffer is genuinely thin, raise KH with potassium bicarbonate and pH follows it up. Crushed coral or aragonite in the filter does the same slowly and self-limitingly. The CO₂ calculator converts pH and KH into a CO₂ number so you can dial in the injection rate against the target band.