Because from experience I can say it doesn't hurt your system as long as the salinity, ph
and temps are the same. As for my reasoning, if the fish are dying due to high nitrates, the quicker they are taken down the better. If you are using the same salt, the temps are the same, the ph
is the same and the salinity is the same, how does that hurt the "chemical make-up"? Here is a link to when I had my nitrate problems.
Also, just because I have advisor under my name doesn't mean I'm always right. I'm still learning lots all the time. I only posted to help this person because I've had experience with fixing a high nitrate problem. This is how I did it with no ill effects to fish/coral/inverts.