I think it';s the salt that maintains the ph with the additives and elements that are in it. I used pool filter sand in my 125g and my ph is around 8.3.
I just out fresh plain sand in my new tank and the ph is at 8.3.
180g- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18