Catit Genuine Fountain Filters for 3L Flower & Fresh and Clear Fountains (12 Pack) (£14.84 / £14.02 max S&S)

A 12 pack of genuine Catit fountain filters is £16.49 at Amazon, which works out at about £1.37 per filter.
They are suitable for the 3 litre Catit Flower Fountain and the Fresh and Clear fountains only, so check your model before ordering, as the listing says they will not fit other fountains.
Catit recommends replacing the filter once a month, so a 12 pack is a full year of filters. They help keep the water fresh by trapping stray hairs, sediment and debris, so the fountain stays cleaner between washes. A handy one for any cat owner with one of these fountains.
They are rated 4.7 out of 5 from over 46,000 ratings on Amazon and were in stock at the time of posting. Sold by Amazon. Free delivery for Prime members or on qualifying orders.