Hello! The pH of my tank recently dropped to 6.0, so I did daily water changes to try and bring it up. The best I could get was a pH of 6.8, so I went and bought some crushed coral. I put it in the 2 media containers that were included with the Emperor 400 filter, and was wondering how often I should replace it. Right now it has been in for 4 days and the pH is perfect. Should I just replace it when I see the pH go down?