I agree with all of the above unless livebearer's or other fishes being kept enjoy water with more hardness.
I have used crushed coral in the past, and found that when placed in the filter, in the path of water being returned to the tank, It can get covered over time,,,with silt,mulm,bacteria,algae,dirt,etc, and then becomes less effective at doin what you want it to do.
I might keep this material clean,or replaced with some regularity depending on it's condition.
If these are soft water fishes being kept, I would not worry bout the drop in pH which could be controlled if needed, by smaller ,more frequent water changes as opposed to single ,Larger water change. Opinion's vary.