If you do larger water changes often (at least weekly) then the paramaters are closer to t he tap water you are adding. Discus breeders do 100% daily. My disscus just laid eggs, so I must be doing something right...
As far as adding salt--1/8 tsp/week isn't going to get you anywhere. It takes roughly a cup of salt/5g to raise your SG by .005. If your goal is eventual SW for these fish, you may never get there. As long as you don't raise your SG faster than .002/week, you should be fine.