100 g calibration weight for precision scales

The first thing to weigh in an aquaponic system is the feed, because it is what supplies the plants' nitrogen. The FAO manual on small-scale aquaponics sets the daily ration at 1 to 2% of the fish's body weight above 50 g, and links the feed to the cultivated area: 40 to 50 g per square metre per day for leafy vegetables, 50 to 80 g for fruiting vegetables. With its ceiling of 20 kg of fish per cubic metre, a 200 L tank holds 4 kg of fish, fed 40 to 80 g per day, enough to supply roughly 1 to 2 m² of leafy vegetables. A badly weighed ration upsets the whole system: too much feed loads the water with waste, too little starves the plants. Weighing the feed ration every day, on scales checked against a 100 g weight, keeps this ratio under your eye.

Next come the correctives, whose doses are small. New Mexico State University aims for 2 mg/L of iron in the water of an aquaponic system; with a DTPA chelate containing 11% iron, that works out at 18 g per 1,000 L, so 3.6 g for a 200 L tank. To raise alkalinity by one German degree (1 °dKH), you need 36 g of potassium bicarbonate per cubic metre, a little over 7 g for 200 L. When weighing only a few grams, a scale that drifts by 5% throws the dose off by nearly 0.2 g with each addition, and the error accumulates over the weeks in water that is only partially renewed.

The right routine comes down to three steps, which you repeat before each session: place the 100 g weight on the scale before a series of weighings, note the deviation if there is one, and recalibrate if the reading drifts from the expected value. For quantities below the scale's resolution, a stock solution takes over. The doses themselves are decided from a measurement: a carbonate hardness test for bicarbonate, the appearance of the young leaves for iron. The article on iron deficiency in aquaponics details these signs, and the one on pH regulation explains the use of bicarbonate.