Water quality: oxygen, pH, temperature, ammonia
The four measurements that explain most fish deaths, how to take them with a simple kit, and what to do with the result.
6 min read
Why these four
A fish lives in its water as we live in air. When it dies without any sign of disease, it is almost always the water: too little oxygen, too much ammonia, a temperature drop, a pH drift. Four measurements explain nine problems out of ten.
Dissolved oxygen
Oxygen is produced by day by the algae and consumed at night by algae, fish and mud. It is lowest just before sunrise. Measure it at 5 or 6 a.m., never at noon.
| Result | Reading | Action |
|---|---|---|
| > 4 mg/L | Comfortable | Nothing to change |
| 2 – 4 mg/L | Tight | Reduce the ration, stop fertilising, check again tomorrow |
| < 2 mg/L | Danger | Circulate or aerate the water tonight, do not feed |
Thick, dark green water announces a night-time oxygen crash: there are too many algae.
Ammonia
Ammonia comes from waste and uneaten feed. It turns toxic as pH and temperature rise. If the fish eat less, swim at the surface and oxygen is fine, think ammonia: halve the ration and exchange part of the water.
Temperature
Tilapia slows below 22 °C and suffers below 20 °C. The short rains and harmattan nights can chill a shallow tank. Shade, a depth of at least one metre and a reduced ration on cold days limit losses.
pH
Between 6.5 and 8.5 all is well. Below 6, lime the pond between cycles; above 9 in late afternoon the algae are too active and ammonia becomes more dangerous.
The "Water diagnostic" tool uses these thresholds and gives a traffic-light verdict from your readings.