sand turning drk brwn to blk

Question
HI I have a 55 gal saltwarter tank its been up and running for about 75 days after I purchased a feather duster and a sand sifting starfish I noticed my sand turning drk brwn to blk in certtain areas where the sand is I have a marine land 350 bio wheel filer with 4 filters in it I did a 12 gal water change recently and changed out all four filters faithfully once a month can you please give me some idea as to why this is doing this and how to get rid of it I have 30 lbs gravel and 40 lbs live sand about 35 lbs of live rock water is crystal clear I have a clown fish and a yellow tail blue damsel in the tank as well as a hation anemone and several crabs  
         thank you
         Andrew

Answer
Hi Andrew,

What you are experiencing within the layers of your live sand is perfectly normal.  To make a long story short, this is the aerobic and anaerobic bacteria's competing with each other; some species will die off others will survive. This is all part of your aquariums biological processes that are constantly taking place. Provided your water parameters are adequate there is no cause for alarm. With time these dark colors in your live sand will start to fade out.

Sincerely,
David