Fish live in their own waste, and without a filter it builds up quickly in a home tank. An internal filter sits in the water, draws it through a sponge to catch uneaten food and debris, and keeps it moving so there are no stagnant corners.

This one does four jobs. It filters through a removable sponge cartridge, pumps, circulates the water and adds oxygen through an air hose with an adjustable valve. Take the filter box off and it works as a plain submersible pump. Flow is rated at up to 500 litres an hour on 6W, a knob on the body turns it down, and suction cups hold it to the glass.

• Flow: up to 500L/H
• Power: 6W
• IP68-rated and fully submersible
• Removable sponge filter cartridge
• Includes a 13mm water nozzle, air nozzle with regulating valve and air hose
• ABS plastic body

Worth knowing: the listing does not say what size of tank it suits, so check that up to 500L/H is sensible for your tank and fish; small or slow-swimming fish may need the flow turned well down. Always unplug it before lifting it out to rinse the sponge.