Aquarium filter media
Filter media is whatever sits inside your filter doing the actual work of cleaning the water — foam pads, filter wool and similar materials that trap debris or host the bacteria that break down waste. Different media do different jobs, and most filters work best with more than one type running together.
Mechanical vs biological media
Filter wool and foam pads are primarily mechanical media — they physically trap floating debris and cloudy particles as water passes through, which is why they're the part that visibly clogs and discolours over time. Foam pads, like the ones made for specific filter models, are shaped to fit snugly and are often reusable with a rinse rather than a full replacement.
Biological media works differently: it provides surface area for the bacteria that break down fish waste into less harmful compounds, and it's this bacterial colony — not the media itself — that you're protecting when you clean a filter. Rinsing biological media in old tank water rather than tap water helps avoid killing off that colony.
Keeping media working
Mechanical media like filter wool needs replacing regularly once it's visibly dirty, since a clogged pad restricts water flow and reduces how well the whole filter works. Foam pads can usually be rinsed and reused several times before they need swapping out entirely.
Never replace all of your filter media at once — doing so removes most of the beneficial bacteria in one go and can trigger the same water quality problems as setting up a brand new tank. Stagger replacements so some established media is always left in place.
In this section
Everything below is in stock with us, priced with delivery worked out, and packed and posted by hand in the UK.
Supa Synthetic Filter Wool
25g of synthetic filter wool for aquarium and pond filters, traps fine particles
£3.29
Supa Activated Filter Carbon
Activated carbon filter media for freshwater, marine and pond filtration systems
£5.45
Supa Filter Carbon
150g of activated carbon for aquarium and pond filters, replace every four weeks
£3.35
Marina I25 Internal Filter
A compact internal filter for goldfish or tropical tanks up to 25 litres
£25.29
Fluval U3 Filter Foam Pad
Fluval U3 foam pads that catch large debris as the filter's first mechanical stage
£7.89
Fluval U1 Filter Foam Pad
A foam filter pad made specifically to fit the Fluval U1 underwater filter
£6.09
King British Filter Aid+ (formerly Safe Water)
A live-bacteria water treatment for tropical, coldwater and marine aquariums
£5.41
Supa Condensation Tray
A 48-inch aquarium condensation tray, shielding the hood's electrics from tank water
£11.49
Aquatic Roman Gravel Jet Black
2kg jet black aquarium gravel, inert and colourfast for coldwater, tropical or marine tanks
£3.99
Pettex Aquatic Roman Gravel Natural Lakeland
Natural Lakeland-tone aquarium gravel for coldwater, tropical or saltwater tanks
From£3.99
Common questions
- How often should I replace filter media?
- Mechanical media like filter wool is worth checking every couple of weeks and replacing once it's visibly clogged or discoloured, while foam pads can typically be rinsed and reused for longer before a full swap is needed. Biological media should be replaced far less often, if at all, since it's carrying the bacteria your filter relies on.
- Should I clean filter media in tap water?
- It's better to rinse biological media in water taken from the tank itself rather than tap water, since chlorine and chloramine in tap water can kill off the bacteria you're trying to preserve. Mechanical media that's due for full replacement doesn't need this care in the same way.
- Why is my filter media all one colour after a few weeks?
- A brownish or grey tinge on filter wool or foam is normal and shows it's doing its job trapping debris from the water. It only becomes a problem once flow through the filter is visibly reduced, at which point it's worth rinsing or replacing rather than leaving it to clog further.
Related
Written by Matt Garnett, who runs Giddy Pets and is a lifelong pet owner — not a vet. This is general information, not veterinary advice, and it cannot replace an examination by the vet who knows your animal. How we write these guides.