crushed coral has the ability to aid the ph at a decent level, and in a fish only if used thinly and cleaned regually is ok, however as mentioned above is just pointless in a reef, the stuff can get covered with detritus and cause nitrates and is hard to remove especially if 2" thick or more, id forget about using it at all ronny and just discard it. the fish wont mind nitrate but corals will and so its a no-no for you imo