Getting back on track, magnetism works on lime dissolved in water, if the field is strong enough, because they are negatively charged ions, consequently they are trapped by the field and prevented from moving upstream of it. They form clumps and are then filtered out. This type of system only works efficiently if you have a good filter system that is cleaned regularly, although even without filters, the mains powered systems are effective at seriously reducing limescale in hard water areas.
Simply sticking a couple of magnets around your incoming pipe will not do the job. If you look at commercial systems they tend to be 250mm to 500mm (10" to 20") long for them to be effective.