Pond sludge vacuum, 40-litre tank

You are working on a pond with its inhabitants still inside, and two habits are enough. Move slowly, giving the fish time to move away from the nozzle, and keep an eye on the tank so you can stop and release immediately if one of them enters it. Watch the water level too, because the water you remove does not come back. A thorough session can draw off several hundred litres, and a pond already low at the end of summer can lose enough in an hour to shift its parameters significantly. The article on compensating for evaporated water explains how to top up that volume without causing a shock. Bear in mind as well that the bottom layer harbours part of your bacterial population, and removing it entirely in one session is asking your filter to start again from scratch at the worst possible moment. Working in strips over two or three weekends protects both the water level and the biological balance, and gives you time to see how the water responds between sessions. This precaution matters most on a heavily stocked pond, where the margin for error is slim.

What comes out of the waste pipe is water loaded with organic matter and fish waste. Direct this pipe towards a border, the base of a tree, or a row of vegetables rather than a drain, since you are turning a maintenance chore into a fertilising irrigation, which is precisely the loop we aim for in aquaponics. The waste pipe works by gravity and must therefore run downhill all the way to its destination, which means choosing the area to be watered before positioning the unit rather than afterwards. A border situated higher than the bank is out of reach, and a pipe extension will not change that.

You receive the motor unit and its tank pre-assembled, the 4 m suction pipe, the 2.5 m discharge pipe, the universal suction nozzle with its tube, the filter element, and the mounting bracket. The power cable measures 7.5 m. The unit weighs 14 kg and measures 39 cm wide by 82 cm tall, making it a permanent fixture rather than something you put away in a cupboard. The manufacturer's warranty runs for 3 years. Plan a sheltered storage spot from the moment of purchase: a unit of this size left in a corner of the garden will gradually fill with leaves and deteriorate faster when idle than when in use. A garden shed, garage, or simple storage box is sufficient, provided it is frost-free.

The unit is used twice a year on most garden ponds, once after leaf fall and once at the spring restart. These two sessions are not equal in importance. The autumn pass removes organic matter before it ferments under ice, and it is the more useful of the two, as it prevents oxygen depletion over winter. The spring pass prepares for the season and is carried out earlier than the filter restart, while the water is still cold and the fish are barely moving. Between the two, whatever settles is broken down biologically, that is the job of the bacterial filter media placed in the filter. A heavily shaded pond surrounded by deciduous trees will, however, require a third pass during autumn, otherwise the accumulated layer exceeds what a single tank can process in one day.

A pond vacuum stores clean or it won't start next time. Drain the tank completely after use, rinse it with clean water, then remove the filter element and run it under the tap until the water runs clear. Both pipes can be rinsed simply by letting them run full from a high point, which flushes out any silt left in the folds. Leave everything to dry thoroughly before storing, because a tank sealed over a residue of silt will develop, within a few days, an odour you will not forget come the following spring. Store the machine in a frost-free place: water trapped in the seals and the pump is what causes them to split. The filter element should be replaced when rinsing no longer restores its appearance, usually after several seasons, and it is the only genuine wearing part on the machine.

A vacuuming session disturbs two balances, the water level and the bacterial population, and the days that follow are worth keeping an eye on. Make up the removed volume with water at room temperature rather than straight from a pipe in full sun, and spread that refilling over time if the loss was significant. Then run an ammonia test and a nitrite test within 48 hours, as this is the point at which a depleted filter shows up in the water before it shows up in the fish. Reduce feeding for a few days as well, to give the bacterial population in the filter time to resume the work that the bottom layer was doing. A pond vacuumed thoroughly will usually return to equilibrium within one to two weeks, and it is precisely during this window that overfeeding comes at a steep price.