Water parameters

What to measure

Clear water is not necessarily healthy water: ammonia and nitrite are invisible. Test kits, as strips or liquid reagents (more accurate), are part of the basic equipment. OATA advises testing at least once a week, and more often when the tank is new or after new fish arrive.

pH: acidic or alkaline

pH measures the acidity of water on a scale from 0 to 14: 7 is neutral, below that the water is acidic, above it is basic (also called alkaline). The scale is logarithmic: water at pH 6 is ten times more acidic than water at pH 7, and a hundred times more than water at pH 8.

Each species has adapted to a specific environment: the acidic, very mineral-poor rivers of Amazonia for many tetras, rather hard water for the livebearers of Central America, the very alkaline lakes of the African Rift for the cichlids of Malawi and Tanganyika.

GH and KH: hardness

GH (general hardness) measures dissolved calcium and magnesium ions: it is what makes water “hard” and leaves limescale. KH (carbonate hardness, sometimes called alkalinity) measures carbonates and bicarbonates. They neutralise the acids produced in the tank, notably by nitrification: with a very low KH, the pH can crash suddenly.

Aquarium tests and Aquarius profiles give hardness in German degrees (°dGH and °dKH). Water reports may use other units: French degrees (°f) in France, or milligrams per litre of calcium carbonate (mg/L CaCO₃, also called ppm) in many other countries. One German degree equals 10 mg/L of calcium oxide, or 17.8 mg/L of calcium carbonate, and one French degree 10 mg/L of calcium carbonate. So 1 °dGH ≈ 1.78 °f ≈ 17.8 mg/L CaCO₃: a hardness of 200 mg/L CaCO₃ (20 °f) is about 11 °dGH.

CO₂, KH and pH

Dissolved CO₂, released by the breathing of fish and bacteria or injected for plants, forms carbonic acid and lowers the pH. When the KH is known, the pH therefore gives an estimate of CO₂: CO₂ ≈ 3 × KH × 10^(7 − pH), in mg/L with the KH in °dKH.

According to Tropica, “easy” plants do without added CO₂, although a little (3 to 5 mg/L) is better than nothing; “Medium” plants need 10 to 15 mg/L and the most demanding 15 to 30 mg/L. To follow CO₂ without calculating, a drop checker filled with a 4 °dKH reference solution turns green between about 25 and 35 mg/L.

Adapting the water, or the fish

The simplest and most stable approach is to choose fish suited to your tap water. Hard, alkaline water suits livebearers, rainbowfish or African cichlids; soft water suits tetras, rasboras and corydoras. The simulator filters the catalogue precisely by your pH, GH and temperature.

If you are set on species from different water, change it gradually and prepare the water for changes in the same way:

• Soften: mix tap water with reverse osmosis or demineralised water, measuring the resulting GH and KH. Pure reverse osmosis water has no buffering capacity left.
• Acidify gently: peat in the filter, roots and catappa leaves release humic acids that lower the pH and tint the water amber.
• Harden: coral sand or limestone rocks (they fizz when vinegar is dropped on them) raise the GH, KH and pH.
• Avoid “pH down” or “pH up” products used on their own: without acting on the KH, they cause sudden swings.