ok here we go,
i used to look after 5 malawi tanks with wite agronight sand and coral,
i found that the coral and sand would go brown after a couple of days, i was doing daily gravel cleans as it was in a shop and the boss wanted it to look clean at all time (fair enough) once a week the sand was fully done,
RE the coral well what i was doing is once a week or so just turning the coral over so the brown wasnt in the direct light, about 3-5 day cycle, this also ment that the tank didnt get boaring...
hope this helps a bit