Thanks for reply.
I do not have artificial life rock in my tank. I’m using RO (purchase from supermarket) though. In the past I do not have this issue where my KH between 8 and 9. My pH is 8.1 calcium 450, magnesium 1350. Whenever I change water around 20 to 30% KH drop to just 17, where before it was 19 initially. Now it increase to 18
I’m really surprise with this result where chemically it doesn’t makes sense. Where calcium and ph is in acceptable range...now I feel even I change water, it will be waste of my effort
Possible for a dying coral rock leach back carbonate hardness?
Appreciate for any help and suggestion.