AMTRA Digital pH Tester ATC electronic pH meter

Remove the cap, rinse the electrode with reverse-osmosis or distilled water and gently dry it with absorbent paper. Switch the device on with the ON/OFF button, immerse the electrode in the water to be measured and stir gently for about thirty seconds, until the digits 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 the reading, switch the device off, rinse the electrode and replace the cap; if you do nothing, the device switches itself off after 5 minutes.

Switch the device on, immerse the electrode in the 6.86 buffer, at 25 °C at most, and hold the CAL button for 3 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 4.00 buffer, hold CAL again for 3 seconds, then press once more while 6.86 is flashing, and the screen moves on to the second point, shown as 4.01. For a second point at 9.18, better suited if your water is above pH 7, press twice instead of once. A measurement in a buffer solution then lets you check the result.

Recalibrate after a long period without use, after very frequent use, and whenever you need a very reliable reading. Avoid holding down the CAL key for no reason, as the meter would go back into calibration mode. An electrode left in the air dries out, and its readings then become slow and unstable: soaking it for a few hours in reverse-osmosis water is generally 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 drifts down slowly from one week to the next. The target range is 6.4 to 7.5, a compromise between the activity of the nitrifying bacteria, which is highest between 7 and 8, and the uptake of iron and phosphorus by the roots, which is better below 7. A weekly reading to two decimal places will show you this drop well before it troubles the plants or the fish. The guide to regulating pH in an aquaponic system describes the corrections available.

Also measure your tap water once, separately. Its value tells you what each top-up or water change adds to the system, and it explains some of the differences you later notice in the tank. In hydroponics, where the nutrient solution is often checked every day, the same device is useful for checking the pH after each addition of pH corrector.