Freshwater Aquarium Water Chemistry: The Overview

How the freshwater parameters fit together: ammonia, nitrite and nitrate; pH governed by KH and CO2; GH versus KH; and why stability matters more than chasing perfect numbers.

Water chemistry can feel like a long list of disconnected numbers, but the parameters are tightly linked, and understanding how they push and pull on each other is what separates a stable tank from a constant battle. The guiding principle from water-quality science is that there is no single correct value: the right parameters are determined by the needs of the animals you keep. This overview ties the individual parameters together; see the dedicated guides for each one in depth.

The nitrogen waste series

Fish and decaying food produce ammonia, which the biological filter converts to nitrite and then to far less toxic nitrate. Ammonia exists in two forms: un-ionised NH3, which is roughly 100 times more toxic to fish than the ionised NH4+. Crucially, the toxic NH3 fraction rises as pH and temperature increase and falls slightly as salinity increases, so the same total-ammonia reading is more dangerous in warm, alkaline water. Damage to fish tissue can begin above about 0.05 mg/L of un-ionised ammonia, and nitrite is toxic from as little as 0.10 mg/L.

pH, KH and CO2

pH measures how acidic or basic the water is on a 0-14 scale, with 7.0 neutral; most freshwater fish are comfortable between about 6.5 and 8.0. pH is not a free-standing value, though. It is held in place by carbonate hardness (KH), the water's buffering capacity. Low KH lets pH swing easily and even crash, while higher KH resists change. Because nitrification and dissolved CO2 both add acidity, pH tends to drift downward over time, and a healthy KH neutralises that acid. As a rule of thumb, keeping KH above about 2 dKH avoids dangerous swings.

GH versus KH

These two hardness measures are easily confused. GH (general hardness) measures calcium and magnesium, the minerals fish, shrimp, snails and plants need for biological functions. KH (carbonate hardness) measures carbonates and bicarbonates and governs pH stability. They are independent: water can be high in one and low in the other. One degree of KH equals about 17.9 ppm.

Target ranges by fish type