Preparing your aquaponic system for autumn

Water has high thermal inertia, which gives a false sense of security. A 1,000-litre pond follows air temperature with a two-to-four-day lag, but it follows it nonetheless, and once the average night-time temperature is consistently below 10 °C, it will not recover.

Freshwater fish are ectotherms; their metabolism depends directly on water temperature. Between 20 and 12 °C, digestion slows considerably, and below 10 °C most temperate species almost stop feeding altogether. This is not a sign of illness but a perfectly normal period of dormancy.

The same cooling affects the bacteria in the biofilter, with a useful lag worth knowing about. They remain active longer than fish appetite lasts, which gives you a comfortable window if you reduce feeding rations before the water drops too low. The nitrogen cycle does not stop, it simply slows down.

A daily reading taken at the same time of day is worth more than a highly accurate reading taken at random. In autumn, the gap between early morning and mid-afternoon can easily reach 3 to 5 °C in a small volume, and it is the morning reading that determines whether to feed.

A remote-probe thermometer left in place avoids opening the system every day. Log the reading in a notebook or spreadsheet, because it is the trends over seven days that matter, not isolated spikes. Two cold days followed by a mild spell are no reason to stop feeding; a full week below 12 °C is.

Also remember to take your reading at the bottom rather than the surface if your pond exceeds 60 cm in depth. Thermal stratification is minimal in a well-circulated system, but it does exist in poorly circulated ponds, and your fish spend autumn in the lower layer.

The most reliable rule is to follow appetite rather than the calendar. Feed the amount consumed in two to three minutes, then remove whatever remains. If pellets are still floating after five minutes, the next day's ration should be reduced.

In practice, feeding typically goes from 1.5 to 2% of body mass per day in summer down to around 0.5% at 14 °C, then to one feed every two or three days below 12 °C. For 10 kg of fish, that means dropping from 150 to 200 grams daily to around fifty grams, then to almost nothing.

This reduction doesn't starve anyone, it respects the animal's physiology. Undigested food sits in the digestive tract and ferments, causing far more serious problems than a seasonal fast. We cover this point in more detail in our article on animal welfare.

Below 14 °C, a pellet rich in animal protein becomes difficult to digest. Aquarists who keep ponds have long used what are known as wheatgerm foods, whose starch is better assimilated at low temperatures, with a reduced protein level that typically sits around 25 to 30% compared with 40 to 45% during the main growing season.

The switch should be made gradually, over one to two weeks, by blending the two foods in decreasing proportions. An abrupt changeover disrupts the intestinal flora and frequently results in poorly formed faeces, visible in the settling chamber a few days later.

At this time of year, opt for a floating pellet, it allows you to see at a glance what has not been eaten and to adjust the quantity the following day. Our pond fish foods clearly distinguish between these seasonal formulations.

Nitrifying bacteria work best between 17 and 34 °C. Several aquaculture sources place the loss of efficiency at around 50 % near 18 °C compared to the optimum, with a very marked drop below 10 °C. They do not die, however, they enter dormancy and restart in spring.

In practical terms, your system processes less ammonia per hour at the very moment when the slightest feeding error produces more of it. This is precisely why rations should be reduced before the temperature drops, not after.

Avoid cleaning the biological media in autumn under any circumstances. Rinsing it with tap water, which is chlorinated, destroys a population that will not re-establish itself for several weeks at that time of year. If rinsing becomes unavoidable, do it in a bucket of system water, never under a tap.

A handful of fallen leaves in a 1,000-litre pond seems harmless enough. As they decompose, they consume oxygen, release tannins that turn the water brown and slightly acidify it, then load the settling chamber with a fine matter that is difficult to remove.

The most effective solution is a mechanical one. A net with a 10 to 20 mm mesh stretched over the pond from the very first week of leaf fall retains most of them, provided it is taut enough not to dip into the water. A slack net turns into a debris trap that eventually releases what it was holding.

Remove the build-up every two or three days during windy periods. If leaves have already settled on the bottom, remove them with a wide-mesh landing net rather than by draining, as a large water change at this time of year inflicts a thermal shock that is entirely avoidable.

The mechanical filter, for its part, must be clean before winter sets in. Sludge that has built up over the summer continues to mineralise slowly and consumes oxygen, precisely where your system needs it most.

The right approach is to rinse the foam media and purge the settler in late September or early October, while the water is still above 14 °C and the bacteria can quickly replenish what has been removed. Always rinse in a bucket filled with pond water, never under the tap.

Keep the two stages clearly separate in your maintenance routine: the mechanical filter needs cleaning regularly, the biological filter as rarely as possible. Water that turns cloudy after an autumn clean is almost always a sign of over-zealous rinsing, a situation we cover in detail in our article on cloudy water.

Cold water holds more dissolved oxygen than warm water, which might suggest that aeration becomes less important in autumn. The demand does indeed fall compared to summer, but it never disappears entirely, and keeping the aerator running remains one of the most important tasks of the season.

Several consumers continue working as the water cools. Leaves and plant debris breaking down, sludge settling on the bottom, the bacteria in the biofilter, working more slowly but not stopping, and the fish themselves, whose metabolism is slowing: all of them draw from the same reserve. There is also a purely mechanical effect: a pond covered with a tarpaulin or sheltered inside a greenhouse exchanges far less gas with the open air, since both the surface area in contact with the atmosphere and wind-driven agitation are reduced.

Keep the air pump running throughout winter, reducing the flow rate slightly if needed. On an outdoor pond, an air pump kit sized for the volume, complete with pump, pipes, and diffusers, will see out the season without intervention. In late autumn, raise the diffuser to mid-depth rather than leaving it on the bottom, because a diffuser placed right at the base circulates the coldest layer upwards and equalises the temperature throughout the volume, whereas the fish are actively seeking that lower, more stable zone in which to overwinter.

Temperature is not the only limiting factor. With around ten hours of daylight per day, most leafy vegetables slow down considerably, regardless of the heat available. In temperate Europe, this threshold is crossed towards the end of October and does not recover until mid-February.

Summer crops still in place, tomatoes, cucumbers, basil, will not make up the lost ground. It is better to remove them than to leave them taking up space, as they continue drawing nutrients without producing anything. Clearing them also frees up light for cold-season crops.

Focus on hardy leafy vegetables, lamb's lettuce, spinach, winter lettuce, chard, kale, watercress. Our selection of 15 plants to grow highlights those that tolerate the colder season best.

Autumn offers the last comfortable opportunity to work on plumbing and electrics. Remove the pump, open the strainer, pull out any fibres wrapped around the shaft, and check the impeller. A clogged pump draws more power and overheats, two faults that will eventually cause it to fail in the middle of winter.

Next, check the frost-vulnerable points: above-ground pipes, fittings in dead-end positions, exposed valves. Any section of standing water in an outdoor pipe will freeze, and a cracked pipe is usually only discovered at the thaw, by which time the pond is already half empty.

Finally, check the drip loop on every electrical cable and make sure there is a 30 mA RCD on the circuit. This detail costs very little and prevents accidents at the time of year when moisture levels are at their highest.

The first mistake, by far the most common, is feeding out of habit. Pellets still floating after five minutes are a warning sign, not a minor detail. They will sink, break down, and generate pollution that a sluggish biofilter will be unable to process.

The second is the autumn deep clean, carried out with good intentions at entirely the wrong moment. Clearing the settler, rinsing the foam pads, and scrubbing the biological media all on the same November weekend is effectively restarting a cycle in the middle of cold weather.

The third is more subtle: switching off the air pump or the water pump to save a few pounds. Cutting the circulation deprives the biofilter of oxygen and kills it within days. All three pitfalls are among the common mistakes we have documented.