EHEIM VAC40 sludge vacuum for ponds, 40-litre tank

You are working on a pond that is full of water, with its inhabitants still in it, and two habits will protect your fish. Move slowly, giving the fish time to move away from the nozzle, and keep an eye on the tank, so that you can switch off and release at once if an inhabitant has been drawn in. Watch the water level too, because the water you remove does not come back. A thorough session removes several hundred litres, and a pond that is already low at the end of summer can lose in an hour enough to make its parameters shift. The article on topping up evaporated water explains how to put that volume back without causing shock. Finally, bear in mind that the layer on the bottom holds part of your bacterial population, and that removing it entirely in a single session leaves your filter to deal alone with the whole load while the level has dropped. Working in strips, spread over two or three weekends, protects both the level and the balance, and gives you time to see how the water reacts between sessions. This precaution matters most on a heavily stocked pond, where the margin is slim.

What comes out of the discharge pipe is water loaded with organic matter and fish waste. Direct this pipe towards a flower bed, the base of a tree or a row of vegetables rather than a drain, since you thereby turn a maintenance chore into fertilising irrigation, following the very principle of aquaponics, where the fishes' waste feeds the crops. The discharge pipe works by gravity and must therefore run downhill all the way to its destination, which means choosing the area to water before positioning the machine rather than afterwards. A bed that sits higher than the pond bank is out of reach, and an extension pipe will not change that.

The machine is used twice a year on most garden ponds, once after the leaves have fallen and once at the spring restart. These two visits do not serve the same purpose: the autumn one removes the material before it ferments under the ice, and it is the more useful of the two, because it prevents the oxygen consumption of winter. The spring pass prepares the season and is done earlier than restarting the filtration, on water that is still cold where the fish move little. Between the two, what settles is digested biologically, and that is the job of the bacteria pearls placed in the filter. A very shaded pond, bordered by deciduous trees, will however need a third pass during the autumn, otherwise the accumulated layer exceeds what a single tank can take up in one day.

A vacuum cleaner put away with mud in its tank and in its pipes clogs up as it dries. Empty the tank completely at the end of the job, rinse it with clean water, then take out the filter element and run it under the jet until the water runs clear. The two pipes are rinsed simply by letting them run full from a high point, which carries off the mud left in the folds. Then leave everything to dry before putting it away, because a tank closed over a layer of mud develops a persistent smell within a few days and a deposit that is hard to remove. Finally, store the machine somewhere frost-free, in a garden shed, a garage or a storage chest, because water trapped in the seals and in the pump makes them burst when it freezes. The filter element should be replaced when rinsing no longer restores its appearance, generally after several seasons, and it is the only genuine wear part of the machine.

A vacuuming job affects two balances, the water level and the bacterial population, and the days that follow deserve some attention. Top up the volume removed with tempered water rather than straight from a hosepipe left in full sun, and spread the refill over time if the loss was significant. Then run an ammonium test and a nitrite test within 48 hours, as this is when a reduced filtration capacity shows up in the water before it shows on the fish. Finally, reduce the ration for a few days, while the filter's bacterial population takes over the work previously done by the bottom layer. A deeply vacuumed pond recovers its balance in 1 to 2 weeks in most cases, and overfeeding during this period quickly raises the ammonium level.

You receive the motor unit with its tank already 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 is 7.5 m long. The unit measures 39 cm wide by 84 cm high, which calls for a dedicated storage spot rather than a cupboard. The manufacturer's warranty runs for 3 years.