Washed expanded clay pebbles, 8 to 16 mm grade

In your ebb-and-flow aquaponic system, the media bed acts as both support and filter: the particles shed by the fish settle and mineralise in it, and the nitrifying bacteria colonise the porous surface of the pebbles, where ammonia becomes nitrate. The flood and drain cycle therefore takes the place of the timer. A bed filled to a depth of about 30 cm needs 300 L of pebbles per square metre, which is 30 bags weighing 120 to 150 kg dry, whereas an 80 L grow bed is filled with 8 bags. After rinsing, check the pH of your water every day during the first week, as the pH of the pebbles, up to 8.3, can exceed the range of 6.4 to 7.5 aimed for in aquaponics; the guide to regulating pH in aquaponics explains how to correct it.

The size matters as much as the nature of the grow medium. The FAO aquaponics manual recommends, for grow beds, expanded clay pebbles 8 to 20 mm in diameter. Finer ones clog up with the fish's solid waste; larger ones offer less surface area for the bacteria and hold plants poorly. The pebbles in this bag, at 8 to 16 mm, fall within this range. Expanded clay has a colonisable surface area of 250 to 300 m² per cubic metre, of the same order as volcanic gravel, and its lightness reduces the load of a bed placed on a balcony, a terrace or a growing table.

At a depth of 30 cm, an ebb-and-flow bed divides into three layers that do not work in the same way. The top 2 to 5 cm stay dry: they act as a screen against light, limit algae and protect the collar of your plants from rot. The middle layer, about 10 to 20 cm deep, fills and then empties with each cycle, and this is where the roots and most of the nitrifying bacteria develop. The bottom 3 to 5 cm stay permanently flooded, so that fine particles of waste build up there and mineralise before feeding the plants.

This filtering role has a limit, which the same manual places at around 15 kg of fish per cubic metre of water, and it is sized on the feed: allow 40 to 50 g of feed per day per square metre of bed for leafy vegetables, and 50 to 80 g for fruiting vegetables. Beyond that, the solids gradually clog the pebbles and create pockets without oxygen, where the beneficial bacteria give way to others. A mechanical filter or a settling tank placed before the bed then catches most of the waste. A few earthworms added to the middle layer complete the job: they consume the droppings, fallen leaves and dead roots, which delays the clogging of your bed accordingly.

Before filling the bed, rinse the clay pebbles in small batches, in a colander with holes or a sieve, until the water runs clear. The clay dust formed during transport then goes onto the garden rather than into your pump or siphons, and this first rinse water never reaches the fish tank. Once rinsed, the clay releases almost nothing: measured in one part pebbles to one and a half parts water, the pebbles show a pH of 7.1 and a conductivity of 0.4 mS/cm.

This near-neutral pH fits the 6.4 to 7.5 range aimed for in aquaponics. Measurements taken on new pebbles, however, range from 6.8 to 8.3, so you should check the water pH every day during the first week, then every week. A lasting deviation most often comes from hard tap water or a poorly rinsed bed, and it is corrected in the tank water with fish-safe products, such as potassium bicarbonate when the pH drops.

In an aquaponic bed, the clay balls stay in place from one season to the next: you remove the large roots at each harvest, and the worms consume the finer ones. In hydroponics, where nothing breaks down the residues, a full clean is done between two crops, with a 24-hour soak in a solution of grow medium enzymes adjusted to pH 5.5, followed by a rinse with Hydro A and B nutrient solution dosed at 1 ml per litre. Clay balls treated in this way should then be rinsed thoroughly with clean water before going into a bed connected to the fish, since neither the enzymes nor the hydroponic fertiliser belong in their water.

Keep an opened bag closed, in a dry place. Clay does not wear out and lasts several years, but an open bag collects dust and debris that would have to be rinsed out again. For an 80-litre grow bed, allow 8 bags, and count on about 30 bags for one square metre of bed filled to a depth of 30 cm, which is 120 to 150 kg of dry pebbles for the structure to support.