Crushed coral being a larger granular size allows organic material to settle and decay within your substrate requiring more effort to keep clean. Sand does not have this problem and also has the added benefit of providing an area for anaerobic bacteria to colonize if the sandbed is deep enough. I would recommend hands down using sand if you are switching to a reef tank.
Decaying organic material, or detrious, can contribute to a low PH due to the carbonic acid produced but there are likely other factors as well causing your PH to be difficult to maintain at the appropriate level.
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