EHEIM classic 150 to 600 external aquarium filter

The 250, 350 and 600 models come in two configurations. The with taps version adds two double taps that isolate the filter from its pipes during cleaning. The version without taps costs less and suits a tank that you maintain by hand, close to a sink. Finally, the 250 model is also available with taps and loose filter media, so you can start biological filtration straight away.

In an indoor aquaponic system, for example an aquarium topped with a grow bed, the tank of this filter becomes the main support for nitrification. The bacteria attached to its media convert the ammonia excreted by the fish into nitrite, then into nitrate, the form of nitrogen that your lettuces and herbs absorb. The filter also retains solid waste, which breaks down in the tank instead of clogging the roots. Finally, you can direct its outlet pipe towards the grow bed rather than the spray bar: the filtered water rises into it, passes through the roots and flows back down into the aquarium by gravity.

On a 200-litre aquarium topped with a grow bed, the 250 model passes roughly 2 times the volume of the bed per hour at nominal flow, which is fine as long as the water returns directly to the aquarium. If you send the outflow to a grow bed positioned 60 cm higher, the flow decreases with height, since the pump tops out at 1.5 m of head: the 350 model, whose maximum head reaches 1.8 m, then keeps a more comfortable circulation. The guide to building your own aquaponic system explains this circuit between the tank and the crops.

The filter is placed lower than the aquarium, so that its pump head sits at least 10 cm below the water level. It always works upright, and the difference between the aquarium surface and the bottom of the canister must not exceed 180 cm. Fix the intake tube in a rear corner with its suction cups, and the outlet spray bar on the same pane, about 1 cm below the surface. Then cut the pipes as short as possible, with no loops or tight bends, and tighten each fitting with its nut. On versions so equipped, the two double taps are fitted on the pipes, close to the canister.

When the flow from the spray bar drops noticeably, it is time to rinse the sponges. With the double taps, you close them, unplug the unit, then separate the quick-release fittings: the pipes stay full of water and the circuit does not need to be re-primed. Without taps, you drain the pipes and re-prime the filter when reassembling, as for the first start-up. Open the clips, take out the sponges and rinse them lightly, preferably in a bucket of aquarium water to protect the established bacteria. Once or twice a year, also remove the pump impeller and brush its chamber; its shaft is ceramic, so handle it without forcing.

You receive the assembled filter, its tank, its pump head and the grilles that hold the media in place. The box also contains the intake pipe and its strainer, the angled spout, the outlet bar, the suction cups and the hose. The versions with taps and the classic 250 without taps come with a starter set of foams and pads; the versions with taps add two double taps, and the version with filter media adds loose media. The filter is designed for indoor use, in water that does not exceed 35 °C, and it comes with a 2-year guarantee. To connect it to another device or extend a run, the grey flexible pipe for filter and pump is available in 12/16 mm, the diameter of the 250 and 350 models. If your system is only just starting up, the NO2 nitrite drop water test will tell you when biological filtration has taken over.