No..buffer is not salt. It is used to buffer ph swings. Like a shock absorber. Your KH
reading show the amount of buffering your water has.
In reefs, this is commonly added with a calcium supplement when wanting to grow Stoney corals.
Be careful of correcting salinity too fast. Use RO
for evaporation. When doing a water change check salinity. If it needs to be raised, use replacement water with a small amount higher or lower salinity as needed. I emphasize tiny amounts. For example, if your tank is showing 1.023 and you want to adjust over a few days to 1.025, make your replacement water (20% or so total volume) go in at 1.024, then 1.025 next time and monitor. Do not throw raw salt in the sump. To lower salinity, you can do the reverse. As said above you can also adjust salinity up by topping off with saltwater and monitoring every time.