Brita C150 Purity FINEST Cartridge
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If you are in an area with good water, water filtration isn't essential. Of course, if you are in a location with poor water quality use of filtration is necessary to protect your investment. Off-the-shelf domestic filters don't remove charged ions that cause scale. You can always descale your machine but with a good filtration system you won't need to. The Brita C150 Purity is my recommendation as it is designed for coffee machines with class-leading deionising resin technology.