Buried aquaponics kit, 500 or 1000 L

The thermal inertia of the ground naturally steers species selection towards cold water: trout, brook trout, perch. These are table fish, not ornamental species, and that changes your relationship with the pond. The 500 L format is designed to produce up to 30 kg of fish and up to 70 kg of vegetables per year; the 1,000 L format, over 50 kg of fish per year. Our article on cold-water aquaponics details what trout require. These species need well-oxygenated water and tolerate sustained temperatures above 18 °C poorly, a constraint that sinking the tanks underground is specifically designed to address. Think in terms of fish biomass rather than numbers, a reasonable stocking density for a domestic system being around 10 to 15 kg per cubic metre, and introduce them gradually to allow the bacterial colony to keep pace.

You won't manage this system in the same way from one season to the next, and that's precisely what the thermal inertia of the soil allows. In summer, your attention turns to oxygen: water that warms up dissolves less of it, and that's when the air-lift does its most useful work. In winter, your fish slow down, eat less, and you reduce feeding accordingly, otherwise uneaten food breaks down and causes ammonium levels to rise. In spring, everything picks up again, and that's the time to increase the load if you wish. This rhythm isn't read from a calendar but from your own records: three parameters are enough to track it, and it takes only a few minutes a week to measure them. Winter calls for one extra precaution in cold climates: even when buried, an open water surface can freeze over, and it's better to keep one spot clear for gas exchange rather than breaking the ice, a shock that fish tolerate poorly.

The circuit is straightforward and breaks down into three stages. The fish load the water in the first tank. The air-lift moves it into the second, filled with clay pebbles, where bacteria process the nitrogen and where your crop roots take up nutrients. The water returns purified. In an installation of this size, you grow what the grow medium supports well: lettuces, herbs, strawberries, cherry tomatoes. You replant as you harvest, with no dormant season and no soil to prepare. The clay pebble media bed acts as a biological filter as well as a growing support, its porous surface housing the bacteria that convert ammonium into nitrates, which is precisely why it should never be fully rinsed out between crops. The rest of the balance can be read from test results, and our guide on the nitrogen cycle explains what to look for.

Three measurements are enough to keep an established system running, and they are all taken in the fish tank. Ammonium and nitrites must remain undetectable; any positive reading signals a bacterial colony overwhelmed by feeding or by an unnoticed dead fish. pH is kept between 6.8 and 7.2, a compromise between what suits the fish, the nitrifying bacteria and the plants, and it should be corrected slowly, never all at once, since a sudden swing does more harm to fish than a value slightly off the ideal. KH is a useful complement to these readings: too low a buffering capacity allows pH to drift from one day to the next without giving you any reliable picture. During the cycling phase, aim for a reading every 2 days over 4 to 6 weeks. Record them, because a series of values tells you what a single measurement cannot, and it is that series that will show you when the colony is established and when the first fish can be introduced.

The air pump is the only active component, and it runs dry, so maintenance amounts to very little. Its diaphragms wear out and lose output after one to two seasons of continuous operation, something you notice from weaker bubbling in the column rather than from any particular noise. Keep it sheltered from rain and above water level, and fit a non-return valve on its outlet so that a power cut does not draw water back into the pipe. The diffusers should be rinsed once a season, as pores blocked by limescale force the pump to work harder without moving any more air. Keep a spare diaphragm in stock, since stopping the air-lift stops both circulation and oxygenation simultaneously, the two functions on which your fish depend.

Both versions sold here are self-assembly kits, supplied with a guide and video tutorials. The groundwork takes more effort than the assembly itself: you need to dig out the space for both tanks and ensure they are properly levelled. Factor that work in separately, decide whether you'll use a spade or a mini-digger, and bear in mind that a compacted sand bed beneath each tank and even backfill all around will prevent the stresses that can distort an HDPE wall. The kit is 95% French in origin, and the warranties cover the pump for two years, the system for five, and the tanks for ten. Plan for two people on installation day, an empty tank is awkward to manoeuvre alone at the edge of an excavation, and allow half a day for filling and start-up before introducing anything living.