In article , Alan Silver wrote:

I'm contemplating a cichlid tank, but have very soft water. My LFS
suggested using a crushed coral substrate and rocks of the sort that you
see in marine tanks as these would help harden the water.

These may harden the water somewhat, but don't expect miracles. A
Cichlid "Salt" or buffer mix will work better.

A simple test is to put a few drops of vinager on the rock. If it
fizzess then the rock probably contains calcium carbonate. Calcium
carbonate will harden the water.