You don't have a problem, in fact your tank has grown out of the "water changes are enough" phase. Your salt choice mimics your chosen parameters very well. The reason I like to do this is if you ever have to do a large WC, the new water matches so you aren't screwing around trying to make it match. I like to do one or two 50% changes a year, that way if anything is out of balance it can be corrected easily.