Aquaponics balcony greenhouse, 3.5 m²

The 300 kg of the empty greenhouse is distributed across its footprint and presents little difficulty; it is the 400 kg of water added afterwards that requires a clear answer. Moving it a few metres is still possible before the water goes in, but once the pond is full it becomes a dismantling job. Choose your final position before commissioning, taking into account sunlight, access to a power socket, and a water point for top-ups. Also make sure you can walk around the greenhouse, or at least reach both sides, for harvesting and for opening the pond. If you add the floor and the growing mounds, add their 200 kg to the calculation too, a figure that is easily overlooked because it is installed after the greenhouse rather than alongside it.

The difference from a kitchen garden becomes clear after a few weeks. In a kitchen garden, you make every decision: when to water, what to fertilise, when to amend the soil. Here, you only control one thing, feeding the fish, and everything else adjusts on its own. Your thirteen towers will receive exactly what the 300 L pond produces, no more, no less. That means an under-stocked greenhouse produces pale plants, while an over-stocked one causes ammonium levels to rise before the bacteria can keep pace. On a small volume like this, that balance makes itself felt more quickly than on a larger installation, which is demanding at first, but an excellent way to learn. A weekly drop test is all it takes to stay on top of it. The three values to check are always the same: ammonium and nitrite, which must remain undetectable, and pH, which should sit between 6.8 and 7.2, a compromise between what suits the fish, the bacteria, and the plants.

A 5,000 L/h pump circulates water from the pond to the top of the towers, from where it flows back down by gravity through the roots. An electric ventilation unit at 300 m³/h moves air beneath the glazing. The whole system runs on 60 W from a standard 230 V 50 Hz socket: no need to upgrade your consumer unit to install it. The circuit runs continuously, including at night, because the bacteria that feed your plants do not stop either. Allow for a protected outdoor socket, and if your supply is prone to outages, bear in mind that a prolonged cut will deprive the towers of water well before the fish are affected, the roots have no reserve above the pond. A pump of this flow rate should also be rinsed once a season, as its strainer traps detached roots and fine particles of grow medium that will otherwise gradually reduce flow to the towers at the end of the line, with no other sign showing itself before the plants begin to struggle.

The 300 L HDPE pond is the engine of the entire greenhouse. The fish load the water with nitrogen, bacteria convert it, and the thirteen towers consume it. A volume of this size heats up and cools down faster than a garden pond, which needs monitoring in both summer and winter and determines your choice of species. It is also this volume that limits the fish stocking density, and therefore what your towers will receive: the ratio between the two is the real adjustment in aquaponics, and our guide on the nitrogen cycle explains what is at stake. The pond is topped up with fresh water in line with evaporation losses; this water should be left to stand or filtered through activated carbon when it is chlorinated, as a direct addition affects the bacterial colony before it harms the fish.

This volume is not suited to production fish-keeping, and the sensible approach remains a hardy ornamental stock kept at low density. Calculate your load in kilograms rather than by number of fish, around ten kilograms per cubic metre is already a cautious benchmark for such a small pond, and introduce them in stages to let the bacterial colony keep pace. Species that tolerate temperature fluctuations are better suited here than those requiring water held to within a degree, since a 300-litre pond under glass is more susceptible to daily variations than it is able to buffer them. In cold climates, monitor water temperature closely over winter and reduce feeding as the fish slow down, since uneaten food breaks down and loads the water at the worst possible time.

Three versions are available, and both the installation service and the connected controller can be added as options at the time of ordering. The first few weeks set the tone for everything that follows: the bacterial colony takes 4 to 6 weeks to establish, with an ammonia spike first, then a nitrite spike, before both drop back to zero as nitrates begin to appear. Plan on testing every 2 days during this period, then weekly thereafter, and record your readings rather than just noting them mentally, as a series of values shows how the nitrogen cycle is progressing in a way that a single measurement never can. Our guide to getting an aquaponic system started walks you through the readings that confirm the system is ready for fish, step by step.