Water Chemistry Basics: pH, Hardness and the Nitrogen Cycle

pH — the acid-base balance

pH measures the concentration of hydrogen ions in water on a logarithmic scale from 0 to 14. A pH of 7.0 is neutral. Values below 7 are acidic; values above 7 are alkaline. Because the scale is logarithmic, pH 6 is ten times more acidic than pH 7, and pH 5 is one hundred times more acidic than pH 7.

For a mixed community tank with common tropical species — tetras, rasboras, livebearers, corydoras — a pH between 6.8 and 7.4 suits virtually all of them adequately, even if their natural habitat is somewhat outside this range. Stability within this range matters more than hitting a specific number: a pH of 7.0 that swings to 6.5 overnight causes more stress than a steady pH of 7.3 that never varies.

What causes pH to change?

In a cycled aquarium, biological activity continuously produces carbon dioxide, which dissolves to form carbonic acid, gradually lowering pH between water changes. Organic matter decomposing in substrate also releases acids. Conversely, photosynthesising plants consume CO₂ during daylight hours, causing pH to rise — a phenomenon called the diel pH swing, which in heavily planted tanks with strong lighting can reach 0.5–1.0 pH units between morning and evening.

Polish tap water typically arrives at pH 7.2–7.8. After dechlorination and 24 hours of off-gassing in a bucket, pH often settles at 7.4–7.6. This is a suitable starting point for most common community fish.

General hardness (GH) — dissolved mineral content

GH (general hardness, also called total hardness) measures the concentration of dissolved calcium and magnesium ions. It is expressed in degrees of German hardness (°dGH) or parts per million (1 °dGH = 17.9 ppm).

Most tank-bred fish sold in Polish aquarium shops have been conditioned to local tap water hardness over multiple generations. Unless you are keeping wild-caught specimens or breeding species that require specific water for successful spawning, matching GH exactly to natural habitat values is rarely necessary.

Carbonate hardness (KH) — pH buffering capacity

KH (carbonate hardness, also called alkalinity) measures bicarbonate and carbonate ion concentrations. Its primary aquarium relevance is as a pH buffer: bicarbonates absorb excess acids, preventing rapid pH drops.

A KH below 3 °dKH provides little buffering. In a biologically active tank with CO₂ production and no buffering, pH can drop dramatically overnight — a phenomenon called pH crash — which can kill fish by morning. Maintaining KH above 4–5 °dKH prevents this.

Polish tap water KH typically falls between 6–9 °dKH, which provides good buffering. If you soften water through reverse osmosis (RO) for sensitive species, you will need to add a small amount of remineraliser to restore KH to safe levels before use.

The nitrogen cycle in detail

Fish and invertebrates excrete waste containing ammonia (NH₃). At pH above 7.0 and warmer temperatures, ammonia exists partly as the more toxic un-ionised form (NH₃) rather than the less toxic ammonium (NH₄⁺). Even at pH 7.0 and 25 °C, ammonia at 0.5 ppm total is physiologically stressful.

Stage 1 — Ammonia to nitrite

Nitrifying bacteria of the genus Nitrosomonas oxidise ammonia to nitrite (NO₂⁻). Nitrite interferes with haemoglobin's ability to carry oxygen — a condition called "brown blood disease" in affected fish. Gilled fish are particularly sensitive. Nitrite above 0.25 ppm in a community tank requires immediate action (water change + root cause investigation).

Stage 2 — Nitrite to nitrate

Bacteria of the genus Nitrospira oxidise nitrite to nitrate (NO₃⁻). Nitrate is far less acutely toxic; most fish tolerate levels up to 40 ppm without visible symptoms. However, chronic exposure above 20 ppm stresses immune function, reduces breeding success, and promotes algae growth. Regular water changes are the primary mechanism for managing nitrate.

Where does the bacteria live?

Nitrifying bacteria colonise surfaces — primarily filter media, but also substrate, hardscape, and tank walls. This is why removing or replacing filter media destroys the cycle. When cleaning filter media, rinse it in a bucket of old tank water, never under tap water. Chlorine in tap water kills nitrifying bacteria on contact.

Ammonia — testing and responding

Any detectable ammonia in an established, cycled tank indicates a problem: overstocking, overfeeding, a dead fish decomposing, filter failure, or a recent treatment with antibiotics that killed bacteria. The response is always the same: identify the source, do an immediate 25–50% water change, and retest after 24 hours.

Ammonia detox products (such as Seachem Prime) can temporarily neutralise ammonia to its less toxic ammonium form, buying time during an emergency without stressing the biological filter. They do not permanently remove ammonia — the underlying cause still needs resolution.

Nitrate — the maintenance parameter

Nitrate accumulates steadily in any aquarium without live plants or a denitrifying filter. The rate of accumulation depends on stocking density and feeding frequency. Testing nitrate weekly gives a reliable picture of whether your water change schedule is keeping up. If nitrate climbs above 20 ppm between weekly water changes, either reduce feeding, reduce stocking density, or increase water change volume.

Heavily planted tanks consume nitrate through plant uptake. In a well-planted tank with appropriate fertilisation, nitrate may stay near zero with only weekly 25% water changes.

Oxygen

Dissolved oxygen (DO) is often overlooked but is critical. At 26 °C, fully saturated fresh water holds approximately 8.1 mg/L of dissolved oxygen. Fish and aerobic bacteria both consume oxygen. Inadequate surface agitation causes DO to drop — most visibly on warm summer nights when tanks run warmer and fish gasp at the surface.

Surface agitation from the filter outlet is usually sufficient in a moderately stocked tank. An air stone provides supplementary agitation in hot weather or when treating disease with medication that may reduce oxygen solubility.

Article last updated: April 2026. Continue reading: Compatible Fish Species →