Call your hardness 12 dH (it just makes the maths easier). If you use a mix of half tap water and half pure water it will give a hardness of 6 dH. Three quarters tap and one quarter pure will give a hardness of 9 dH.
But as mbsqw1d said you'll have to use the same mix at every water change to keep the water parameters stable.
But as mbsqw1d said you'll have to use the same mix at every water change to keep the water parameters stable.