Why we test water
Fish breathe oxygen that is dissolved (mixed) in the water, and they have no way to leave bad water. A small change in temperature, oxygen or salt can make them sick. So we measure often and write the numbers down.
Temperature and dissolved oxygen
Use a clean thermometer, keep it in the water for one minute and read it with the bulb still in. Dissolved oxygen (DO) is measured in mg per litre (mg/L), with a test kit or a meter. Warm water holds less oxygen than cold water, and fish also need more oxygen when it is warm. So hot days are the danger time. Many fish want at least 5 to 6 mg/L. Signs of low oxygen: fish gasp near the surface and swim slowly.
Fix it: air pumps, fountains or paddle wheels, less feed, fewer fish, shade.
Salinity: how salty is the water?
Salinity is how much salt is dissolved in water. Open sea water has about 35 grams of salt in every litre (35 parts per thousand). Rivers are nearly 0. Sea fish are stressed in water that is too fresh, and river fish in water that is too salty. A refractometer or a salinity meter gives the number quickly. Rain or river flow can lower salinity near the coast, and hot sun can raise it in shallow ponds.
Clarity and the Secchi disc
A Secchi disc is a white (or black and white) round plate on a rope. Lower it until it just disappears, then lift it until it just appears, and read the depth on the rope. A big number means clear water. A small number often means many tiny floating plants (phytoplankton) or mud. Some green water is good food for young fish, but too much can use up oxygen at night.
Try it: a safe test at home
Fill a clear glass jar with tap water and another with pond water or water with a little soil. Make a white card on a string (your own small Secchi disc) and lower it in each. Which disc disappears at a smaller depth? Also place one jar in a warm place and one in a cool place and read a thermometer each hour. In the 3D free play, set temperature 10, then 30, and note the oxygen number each time.
Key formulas and definitions
- Rough rule: oxygen falls as temperature rises (about 14.6 − 0.28 × T mg/L in this model)
- Sea water salinity ≈ 35 g per litre (35 ppt)
- Secchi depth: big number = clear water
- Safe oxygen for many fish: about 5–6 mg/L or more
Worked examples
1. In the model, oxygen = 14.6 − 0.28 × T. Find oxygen at 15 degrees.
14.6 − 0.28 × 15 = 14.6 − 4.2 = 10.4 mg/L.
2. Find oxygen at 30 degrees. Is it above 6 mg/L?
14.6 − 0.28 × 30 = 14.6 − 8.4 = 6.2 mg/L. Just above 6, so only just safe.
3. A tank holds 200 litres. Sea water needs 35 g of salt per litre. How much salt is needed?
200 × 35 = 7000 g = 7 kg of salt.
4. At 20 degrees, how many mg/L of oxygen less than at 10 degrees (use the model)?
At 10: 14.6 − 2.8 = 11.8. At 20: 14.6 − 5.6 = 9.0. Difference = 2.8 mg/L less.
Common mistakes
- Reading the thermometer after taking it out of the water.
- Testing only in the cool morning and missing the hot afternoon.
- Thinking clear water is always the best. Some green plankton water is good food.
- Moving fish quickly from fresh to salty water without a slow change.