Pocket scale, 500 g in 0.1 g steps or 100 g in 0.01 g steps

The amount of feed given each day is also used to size the crops. The FAO recommends a feeding rate ratio of 40 to 50 g of feed per day per square metre for leafy vegetables, and 50 to 80 g for fruiting vegetables such as tomatoes. A 2 m² grow bed planted with lettuce therefore calls for 80 to 100 g of feed per day, which is the ration for about 10 kg of fish, corresponding to a 500 L tank in the tables of the same guide. Weigh this ration each morning, and you will know whether your fish stock really feeds the area under cultivation.

The weight of the fish changes over the weeks, and the ration with it. A sample weighing every fortnight, noted in a notebook together with the water temperature, shows you the real growth of the stock and lets you adjust the ration before the plants run short of nutrients or the water becomes overloaded. When the water cools, the fish eat less: the weighed ration goes down, and any leftovers at the bottom of the tank tell you that it needs reducing further.

The mineral supplements for a small system are dosed in fractions of a gram. DTPA chelated iron is dosed at 18 g per 1,000 L when aiming for 2 mg of iron per litre: for a 20 L sump tank, that comes to 0.36 g, an amount that only the 0.01 g version can weigh without rounding. Weigh the powder on a tared piece of paper, dissolve it in a glass of water from the system, then pour the solution slowly into the sump tank. For larger additions, such as Epsom salt or potassium bicarbonate, the tenth of a gram of the 500 g version is enough. The hundredth is a matter of readability, not guaranteed accuracy to the hundredth: on 0.36 g, an error of 0.02 g represents more than 5% of the dose, which is still far finer than the whole-gram rounding of a kitchen scale.

A pocket scale gives reliable results provided a few conditions are met. Place it on a flat, stable surface, away from draughts and the vibrations of a pump, then wait for the display to settle before reading the value. Check it from time to time: placed alone on the platform, the 100 g calibration weight should display its stated value, on the 500 g version as on the 100 g version. If the discrepancy grows from one check to the next, recalibrate with the mass specified in the instructions, and do so again after a knock or after transport. Stored with the lid closed, away from the humidity of the room, it will keep its accuracy from one season to the next.