Pocket electronic pH tester, accurate to the hundredth

Remove the cap, rinse the electrode with osmosis or distilled water and gently dry it with a paper towel. Switch the unit on with the ON/OFF button, immerse the electrode in the water to be measured and stir gently for about thirty seconds, until the numbers stop changing. Taking a little water in a clean glass saves you from stirring up the bottom of the tank and disturbing the fish. After taking the reading, switch the unit off, rinse the electrode, replace the cap and store the tester in its rigid case; if you do nothing, the unit switches itself off after 5 minutes.

Switch on the device, immerse the electrode in the 6.86 buffer, at 25 °C at most, and hold the CAL key for a few seconds: the value 6.86 flashes, then the display freezes once this first point is recorded. Rinse and dry the electrode, immerse it in the pH 4 buffer, hold the CAL key again for a few seconds, then press once more while 6.86 is flashing, and the screen moves on to the second point. The 9.18 buffer is then used as a check, or as the second point for water above pH 7, according to the supplied instructions. Finally, a measurement in a buffer solution lets you verify the result.

Recalibrate after a long period without use, after very frequent use, and whenever you need a very reliable reading. Avoid holding the CAL button down for no reason, as the meter would switch back to calibration mode. An electrode left in the air dries out, and its readings then become slow and unstable: a soak of a few hours in osmosis water is usually enough to restore it, and putting the cap back on after each measurement limits this drying out.

In an aquaponics system, nitrification consumes alkalinity, so the pH of a running system slowly drops from one week to the next. The target range is 6.4 to 7.5, a compromise between the activity of nitrifying bacteria, which is stronger between 7 and 8, and the uptake of iron and phosphorus by the roots, which is better below 7. A weekly reading to the hundredth, taken before the fish are fed, shows you this drop well before it affects the plants or the fish. The guide to regulating pH in an aquaponic system describes the possible corrections.

Also test your tap water once, separately. Its reading tells you what each top-up or each water change adds to the system, and it explains some of the differences you later notice in the bed. In hydroponics, where the nutrient solution is often checked daily, the same meter lets you check the pH after each addition of pH corrector.