Slow down...
Why add a buffer to newly mixed salt water?
Test newly mixed salt water. If the KH is below 5, then add a buffer and bring it to 6-7. -- Then, use a better salt water brand.
What is the calcium in newly mixed salt water? If it is under 375 ppm. at SG 1.025, use a better brand of salt mix.
What was the pH, 8.4 or 8.8? There is a difference.
If you are using an indicator dye test kit, it might be impossible to determine any pH over 8.4. If in doubt, get a good quality pH test meter. NOT a pH pen.
50%-60% water changes are quite massive. -- The real point is the water you are adding. From what you have indicated, the salt mix you purchased is highly suited to Pets Mart... Not intended for a serious marine keeper.
Back to the theme of nitrate. Check your: filtration, feeding, live stock or bio-mass.
Go slow. Understand each and everything you are doing with your CAPTIVE marine pets.
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