Chemistry is a mess. Please help.

[LeanneP]

I have a 90 gallon mixed reef tank and for the past month my alkalinity has been low and I can't seem to get my tank stable. I talked to J &l and was told to do a 20 gallon water change but my alk is at 6.1 and my new sw had alk at about 12.5. Won't that be too much of a shock to my tank if the alk goes up to 8.6 from 6.1? Is this the best way to solve this because I am at my wits end. I have a doser and have used the calculator but it has not worked. I think that I may have been dosing too much alk and my ionic balance got screwed up. I have started dosing calcium again and my mag is at 1500. Should I get a different salt with lower alk? Just wondering if using a new type of salt will be hard on the tank too.