Are you asking about products like RO right? Most of the additives I've seen are marketed towards planted tanks and freshwater cichlid tanks. Our salt mix contains most anything we already need in the water. If your PH won't stabilize you might need more rock work or aragonite sand (helps naturally keep the PH high). There are buffers in the salt mix so an easy and good alternative is to do more frequent water changes, that way you can have buffers ready to go without adding anything "extra" to the water.

I don't add anything to my make up water.