Buried aquaponics kit, 500 or 1000 L

The thermal inertia of the ground naturally steers the choice of species towards cold-water fish: rainbow trout, brook trout, perch. These are food fish, raised at low density, roughly four times less crowded than in fish farming. The 500 L format produces up to 30 kg of fish and up to 70 kg of vegetables a year, and the 1,000 L format more than 50 kg of fish, figures that are indicative and depend on the species and how the system is run. Trout need well-oxygenated water and tolerate a sustained rise above 18 °C poorly, a constraint that burying the tanks helps you keep within. You introduce the fish in stages after the biofilter has been started up, and you can read the article on trout in aquaponics for their detailed requirements.

The balance of an aquaponic system is set by the feed you give each day. On the 500 L format, a tank stocked with 15 kg of trout fed at 1% of their body weight receives about 150 g of feed per day. Since one square metre of leafy vegetables makes use of 40 to 50 g of feed per day, and one square metre of fruiting vegetables 50 to 80 g, this ration supports about 3 m² of lettuce. Start with the least demanding crops, lettuce, chard and spinach, and add cherry tomatoes, herbs and then courgettes as the fish load increases. The media bed also acts as the biological filter, its porous surface housing the bacteria that convert ammonia into nitrates; this is why it should never be washed completely between crops. The guide to the nitrogen cycle explains these transformations in detail.

You do not run this system in the same way from one season to the next. In summer, your attention goes to oxygen: warming water holds less of it, and the air-lift, which oxygenates continuously, then comes into its own. In winter, the trout stay active in cool water that the ground keeps from freezing at depth, but they eat less: reduce the ration as the temperature drops, because feed the trout leave uneaten decomposes in the tank. In spring, growth resumes and you can increase the stocking load if your readings stay stable. A tunnel or geodesic greenhouse placed over the growing tank extends the plants' season. Finally, the system tolerates up to 15 days of absence, provided a neighbour calls in to check that the air-lift is running.

The media bed supplied is new: before it can receive trout, it must be colonised by nitrifying bacteria, which takes 3 to 5 weeks, longer in cold spring water. Take your samples from the fish tank, where ammonium and then nitrites are read in succession; as long as either of the two is still measurable, the tank is not ready. Once both read zero and nitrates are visible, introduce only a few trout, then increase the stocking in stages. The pH is set between 6.4 and 7.5 and corrected in small steps, as trout tolerate sudden swings poorly. Test these values every other day during this phase, then once a week. An aquaponics test kit covers these measurements, and starter bacteria speed up the establishment of the colony.

The air pump is the only active part you will have to keep an eye on, and it runs dry, so its maintenance comes down to very little. Its diaphragms wear with running hours and lose output, which shows as weaker bubbling in the column; keep a spare diaphragm in hand, since a stopped air-lift interrupts both circulation and oxygenation. Place the pump out of the rain and above the water level, and fit a non-return valve on its outlet so that a power cut does not let water flow back into the pipe. An annual service can be entrusted to the manufacturer, who will then bring the system up to date. With this annual servicing, the installation's lifespan exceeds 10 years.

The version offered here is a kit that you assemble yourself, supplied with a guide and video tutorials. The groundwork takes more effort than the assembly: you need to dig the pit for the two tanks and plan how to bed them in. Decide whether you will do it with a spade or a mini-digger, and bear in mind that a layer of compacted sand under each tank and an even backfill all around prevent the stresses that deform an HDPE wall. Plan for two people on installation day, as an empty tank is awkward to manoeuvre at the edge of an excavation, then half a day for filling and start-up, before introducing anything living. 95% of the equipment is of French origin.