Water that looks clear can still be carrying dissolved metals, chemicals and dyes that a mechanical filter alone won't catch, which is where a chemical filter media like carbon does a different job to floss or wool.

This is carbon for use in a water filter, working to remove contaminants such as metal ions and dyes that can affect a fish's health and life expectancy, and to improve water clarity generally. Supa recommend at least 1g of carbon per litre of water, rinsing any dust out before use, and packing it either in a fine mesh bag or between two layers of filter wool inside the filter. It's suitable for both aquarium and pond filtration.

• 150G pack of activated carbon
• Removes contaminants including metal ions and dyes
• Use at a rate of at least 1g of carbon per litre of water
• Rinse before use to remove dust
• Suitable for freshwater and marine aquariums and pond filters
• Comes in a resealable bucket

On how often to change it. Supa advise replacing the carbon roughly every four weeks - it stops working once it's saturated, so leaving it in indefinitely defeats the point, even though nothing about it looks different once it's exhausted.

Who it's for: aquarium and pond keepers who want a chemical filter stage alongside mechanical filtration, and who'll actually remember to swap it out on a schedule.