IMO, Salifert tests are better than Nutrifin, though I use a Nutrifin Ca test.
I don't understand how you get 7.5 to 8.0 for a pH test. If it is a colour matching test, you should be able to narrow the pH to 8.0 or some other number. 7.5 pH is very, very low for a reef
As for doing weekly 20% water changes, this is good
Keep it up
However, I don't know the chemistry needs of your tank, and you won't either until you begin testing regularly. The only way of making sure your water changes are replenishing alk, Ca and Mg by the organisms in your tank is to do very large water changes weekly, say 50% or more.
Other than that, I can only encourage you to test for alk and magnesium as well as Ca and nitrate.