As far as i know what reverse osmosis does, is that the water pass through a filter that only water molecules can pass since they are smaller, which means it holds even the minerals the water had, but since we need the water to have a certain amount of minerals, then the filter system adds the miner...

Some multi cartridge RO units have a mineralisation stage after the RO membrane if designed for human consumption. Lab RO units do not. How many stages does your unit have?