I started my new aquarium 3 weeks ago, i let the water cycle until there was 0nitrites 0 nitrates and the ph was right, i then put in a live rock in the tank i got from my friend who had his in his aquarium of 2 years, and added a ocellaris clown fish everything was fine for about 2 weeks then my sand started to get brown spots now all the top layer of sand is all brown and there is brown spots on the aquarium walls. can anybody tell me what went wrong? how can i get rid of this ugliness? Please help!