The cloudiness in a tank that a carbon or biological stage doesn't fix is usually just fine suspended particles - fish waste, decaying food - too small for most media to catch, which is the specific job filter wool does.

This is a fine web of synthetic fibres designed to trap those fine particles without affecting the water's chemistry, suitable for freshwater and marine aquarium filters, including external canister, box and trickle filters. Supa's instruction is to pack it firmly with even coverage across the filter mouth, so particles can't simply bypass it round the edges, and to clean it periodically in clean water, replacing it once it's no longer effective.

• 25G pack of synthetic filter wool
• Traps fine particles without affecting water chemistry
• Suitable for canister, box and trickle filters
• Works in freshwater and marine setups
• Comes in a resealable tub

On when to replace it. Supa's guidance is to clean it periodically and replace it "if necessary" rather than on a fixed schedule - there's no stated lifespan here, so it's a case of checking it visually rather than working to a set interval.

Who it's for: aquarium and pond keepers who want a mechanical polishing stage to clear fine cloudiness that carbon or biological media alone won't catch.