If the nitrates ammonia were high wouldn't the corals show stress before the fish though?Something likely leached from rock to raise nitrates and possibly ammonia levels. You may have also developed a parasite outbreak as rabbitfish has many diots from Mucus spores and likely cryptocaryon (Marine Ich). problem you have with this is that is accelarated stress and not treatable with conventional means such as copper. Rather, you have to reduce any stress the fish will be subject to and as trophants will be dropping to sand to regenerate, Siphon Sand every morning until the rash effect on fish subsides. A high watt UV sterilizer unit may help with this until the parasites lose their life cycle.