Naps small garden aquaponics kit, 500 L pond

The tray measures 1 m by 1 m and 12 cm deep, and its clay pebbles do two jobs at once. They hold the roots in place, and their porous surface houses the colony of nitrifying bacteria that turns fish waste into nitrates the plants can absorb: the tray is therefore both your vegetable garden and the biofilter for the pond. With 12 cm of grow medium, the tray suits shallow-rooted plants such as herbs, salad leaves and edible flowers, far better than fruiting vegetables, which need a large root volume. Aim for basil, coriander, mint, parsley or mesclun, and look at our selection of plants to grow in aquaponics before sowing. Replant as you harvest rather than all at once, so that the plants take up nitrates at a steady rate. Never wash the grow medium completely between crops, as it carries the bacterial colony on which the purification of the water depends.

The three viewing windows change the way you keep an eye on the pond. With a dug pond, you look down from above and see only backs: a fish that is losing weight, rubbing against the walls or staying on the bottom can escape your notice for weeks. From the side, you see the swimming, the flanks and the fins, and you spot a problem before it becomes a loss. The water itself is easier to read from the side, since the first signs of cloudiness show through the window well before they can be seen from the surface. The pond also stays closed by its lid, so a young child can see the fish without being able to reach the water. Clean the windows from the outside each time you pass, with a non-abrasive sponge, as a scratched window stays cloudy for good.

In an ordinary ornamental pond, fish waste builds up and you have to filter, siphon and replace part of the water. In the Naps, the pump sends this laden water onto the tray, where bacteria convert the ammonia into nitrates and the roots take them up, then the water falls back into the tank with most of these compounds removed. You therefore have no fertiliser to add for the plants and much less pond maintenance to do. Your routine comes down to a few tasks: feeding the fish, topping up the level, wiping the viewing windows and testing the water, which takes a few minutes a week once the system is set up. The water is never thrown away; only evaporation and what the plants transpire need to be made up, a few litres a day at the height of summer. The article on topping up water in July and August explains how to top up without shocking the fish.

A volume of 500 L above ground is enough for a stable population but buffers temperature swings poorly, which sets it apart from an in-ground pond of the same capacity. In July, warming water holds less dissolved oxygen just when the fish consume the most of it, and the fountain then comes into its own. In winter, the fish slow down and eat much less: reduce feeding as the water cools, because uneaten feed breaks down and raises the ammonium level just when the bacteria are also slowed. Your tray spends the winter growing lamb's lettuce or baby leaves rather than basil, which cannot tolerate the cold. In a harsh climate, the surface may ice over; keep an open patch of water for gas exchange rather than breaking the ice, as the shock is transmitted to the fish.

Assembly is tool-free and nothing needs to be dug or poured, which leaves you free to choose the location, and to move it later by emptying the pond. First consider how much sun the tray will get and how close you are to a protected outdoor socket, as the pump and the UV unit run continuously in all weathers. A spot that gets the morning sun and a little shade in the afternoon is often the best compromise, because full sun benefits the crops but quickly warms 500 L of water. Also leave room to walk around the pond to feed, measure and harvest, and to reach the three viewing windows. The Naps is made in France and delivered as a kit; if you would rather not assemble it yourself, installation and commissioning by a partner installer is available, with support during start-up.

During the first month, the bacterial colony is establishing itself in the clay pebbles, and you only introduce the fish once it is in place. The readings then cover ammonium and nitrite, which rise one after the other and then become undetectable, while nitrates appear: it is the return to zero of these two readings that allows you to introduce the fish, usually after 3 to 5 weeks. Then keep the pH between 6.4 and 7.5, a compromise between the needs of the fish, the bacteria and the plants, and KH completes this monitoring, since a buffering capacity that is too low lets the pH swing from one day to the next. A reading every 2 days during start-up, then one a week, is enough; note down every value, as a series of readings shows how far the cycle has progressed. Starter bacteria shorten this wait, and an aquaponics test pack covers the four measurements; the guide to starting an aquaponic system properly sets out the steps in order.