Pack of 5 aquaponics water tests: NH4, NO2, NO3, pH, KH

A single reading is hard to interpret. Note each reading with its date, its temperature and the last intervention made on the system, and it is this column of dates that will help you most: a series over 6 weeks shows a trend where a single measurement shows only a number. The practical aquaponics calendar, month by month, also shows which checks to keep up in winter, when the system is running slowly.

This monitoring is mainly used to balance the fish and the crops. The benchmark published by the FAO for a raft or gutter system is 40 to 50 g of feed per day per square metre of leafy vegetables, and 50 to 80 g for fruiting vegetables. If your nitrate rises week after week, the feed given exceeds what your growing area can absorb, and it is that area which needs to be increased. If it stays close to zero once start-up is over, your plants are short of nitrogen and the feed or the fish stocking is too low.

The five tests share the same handling, and it is this handling that makes your measurement accurate. Always hold the dropper bottle vertically, tip downwards, and let the drops fall without an air bubble, with the outside of the tip perfectly dry, because a badly formed drop changes the dose. Rinse the test tube several times with the water to be analysed before filling it, and measure the volume with a syringe rather than by eye. Finally, store the reagents dry, at room temperature and in their original packaging, and let a cold sample come back to room temperature before testing, since cold water slows the development of the colour.

Three of the five tests, ammonium, nitrite and nitrate, are read with a comparator block rather than by placing a single test tube against the colour chart. You fill two test tubes with the water to be analysed, add the reagents to one only, then place both in the block: the tube without reagent acts as a control, so the natural tint of your water, yellowed by tannins or peat, appears on both sides of the comparison and no longer skews the reading. Above each colour field, three pictograms distinguish the uses: a fish for freshwater, a koi for the pond and a starfish for seawater, each followed by a green, yellow or red band that places the value. In aquaponics, you read the freshwater line, or the pond line when the system is installed outdoors.

Each box runs out at its own pace, which you can estimate from the start-up routines. During the first 3 weeks, a daily ammonium reading uses 21 tests, which leaves 29 in the box. At one test a week after that, what remains still covers about 7 months, and nitrite follows exactly the same curve. Nitrate, read every week from the moment the water goes in, uses up its 40 tests in about 9 months. When a bottle is empty, the glassware, the syringe and the colour chart are kept, since only the reagents run out: refills exist for ammonium, nitrite, nitrate, pH and KH.

The reagents in these five tests are not alike: the nitrite one contains acetic acid, the ammonium one sodium hydroxide and hypochlorite, the nitrate one zinc powder, and the KH and pH ones ethanol. Four boxes carry the Danger label and the pH one the Warning label. The rule is nevertheless the same for all five: the water is sampled away from the tank, each reagent stays in the test tube from its own kit, and the tested sample goes down the sink. Keep the boxes together, in a dry place, away from any flame and out of reach of children, keeping the ammonium test apart from any acidic product. When a bottle runs out, its refill can be ordered on its own: ammonium, nitrite, nitrate, pH or KH.