Darlly's Pure-Stream transforms calcium and magnesium into sodium using ion-exchange resins within the filter. Following this process, your water has the same quality as mineral water. This process removes the calcium and magnesium from water source leaving the lime scale in the filtered water suspended where it cannot cling to equipment or surfaces and producing soft water. This is highly beneficial because lime scale deposits are very damaging to the surface and to the internal workings of the spa and often uncomfortable for the user. Especially effective in hard water. Eastern England being most affected, with Lincolnshire and Humberside amongst the worst. This will last you over a year, so great value! Bedfordshire, Berkshire, Buckinghamshire, Cambridgeshire! Derbyshire, Dorset, Essex, Goucestershire, Hampshire, Kent, Leicestershire, Lincolnshire, Middlesex, Norfolk, Northamptonshire, Nottinghamshire, Oxfordshire, Rutland, Somerset, Staffordshire, Suffolk, Surrey, Sussex, Warwickshire & Wiltshire are all effected by very hard water.