When you get something like this, get a different brand of test kit and try again. Sometimes a test kit can go bad. Also test you ro/SI water and make sure this water is 0 before you mixt your salt with it, just to be sure.
I agree with @nanoreefing4fun in that a 50% water change should have reduced nitrates by 50%. So you either have a nitrate level way over what the test kit can read or you have a bad test kit. Perhaps your LFS can do a test for you using a different brand of test kit?
It is possible to extend the range of a test kit by diluting the test sample with distilled water. For example, if you use 1 part tank water to 9 parts distilled water, the mixed water will contain 10% of the nitrate in the original tank water. Then do the test using the mixed water and multiple the results by 10.
I agree with @nanoreefing4fun in that a 50% water change should have reduced nitrates by 50%. So you either have a nitrate level way over what the test kit can read or you have a bad test kit. Perhaps your LFS can do a test for you using a different brand of test kit?
It is possible to extend the range of a test kit by diluting the test sample with distilled water. For example, if you use 1 part tank water to 9 parts distilled water, the mixed water will contain 10% of the nitrate in the original tank water. Then do the test using the mixed water and multiple the results by 10.