Its the crushed coral, change it out as well as anything else you have in the tank that was originally made for saltwater or came from an ocean. I recently bought some substrate that is white gravel and looks just like coral. Ide suggest getting some if you want the same look as before. Sorry to hear about your fish. I know this story all too well cause I did the same thing with some other fish. Just takes experience in loosing a fish to know what not to do next time. Like I said before, you can put some on your filter for buffering. I have to because I use duckweed in my tank for nitrate reduction and it kills my Ph levels. I only use about two tablespoons of coral to buffer and 55g at 6.8
Sorry bout your Aro buddy. And don't let this stop you from keeping fish! Keep trucking! It pays off!
-Don't become an "Energy Vampire"