Limestone (not sandstone), aragonite, and crushed coral is used to buffer Kh but it is a very slow process as it has to dissolve in the water.
First off determine what your fish require. If they require a higher Ph and hardness you will need
1) bicarbonate of soda to raise Ph (will only raise to a max of 8.2) and Kh which is your buffer to keep Ph from fluctuating called Carbonate hardness best if kept between 5 dH and 10 dH
2) Epson Salts to increase your hardness of the water which is called General Hardness - again depends on fish requirements
3) Sea salt - for overall fish health and recovery from injuries and to replace trace elements.
Take a 10 gal container (I used my hospital tank) with fresh tap water and add 1 tsp soda, 1 tsp epson salts and 1 tsp sea salt, aerate for 30 minutes and check your Carbonate hardness and General hardness.
Add more soda and epson salts to get your Carbonate and General hardness at the level you want. No need to increase the sea salt, 1 tsp / 10 gal seems to work for me.
Once you determine the formula you need prepare the solution for the size tank you have in a separate container and slowly add to the tank (over the course of 24 hours).
When water change time comes predetermine how much you are changing ie 30 gal and have prepared the formula necessary and add gradually as your refilling the tank.
Do not add salts when replenishing evaporated water.
Hope this helps.