Some of the terminologies to understand when deciding on what water purifier or filter is suitable for your house:

What is TDSTotal dissolved solids (TDS) comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water.
What is the TDS of drinking water?In terms of Total Dissolved Solids (TDS), in the U.S., the Environmental Protection Agency (EPA) advises against consuming water containing more than 500mg/liter, otherwise known as 500 parts per million (ppm) of TDS, although many health specialists believe that ideal drinking water should be under 50 ppm or lower. It is generally considered safe upto 160-175ppm. The Bureau of Indian Standards (BIS) fixes the upper limit of TDS in drinking water at 500 ppm
Is low TDS water is harmful?Not really, it is generally better to have a low count TDS from drinking water. But there is a limit, minerals so the ultimate goal is not to get a TDS of zero.we also need some minerals. The salt content of blood is slightly inferior to 1%, Anytime you drink water below 1%, you are loosing electrolytes.
What should be the ph value of drinking water?The pH of pure water is 7. In general, water with a pH lower than 7 is considered acidic, and with a pH greater than 7 is considered basic. The normal range for pH in surface water systems is 6.5 to 8.5, and the pH range for groundwater systems is between 6 to 8.5.
WHO report on reverse osmosis?Just about everyone knows that Reverse Osmosis (RO) systems excel at removing water impurities, but few are aware that they also remove the beneficial minerals. In fact, the reverse osmosis process removes 92-99% of beneficial calcium and magnesium.
Health risks from RO / demineralized waterHere's a World health organization report on bad effects of RO / demineralized water