I keep my Alk in between 7.7-7.85, I want to raise it around 8. I had this same issue that your having and thought it was nitrate/phosphate. I couldn't register phosphate on Hanna meter but was getting >5ppm on Nitrate tests, so I got an alge scrubber & ran it along w/ skimmer & I also vodka dose. Can't get any readings on phos/nitrate now. I really think I've been stripping the water too much so I took the scrubber off line today. Come to find out ,I think it was Mg it was reading 1145. It took me about a wk to get it bk up to around 1350 but IMO I think Mg was the issue all the time. It seems like a lot of hobbyist don't test Mg nearly enough and rely on water changes to replenish the Mg. If you don shake the devil out of the salt bucket b'f u do your water changes the Mg will separate or clump in the salt mix & you'll get diverse measurements from batch to batch.

