How can i drop hardness of the tank water?

im testing my water with QUICK DIP 6 IN ONE today it came like this Nitrate 20 Nitrite 0 Hardness 300 !!!! Alkalinity 120 ph 7.5 but look at this hardness WOW how can i take it to be soft again? and now i know why i got white mineral deposits on top of my water. help. I did add some auqarium salt today but i dont think that salt would do that