I've noticed that towards the front of my tank, my sand is turning a brownish color. My tank is 6 months old now so I don't think It could be diatoms unless they could reoccur? It doesn't look like cyano nor algae. I took a picture of it. The iPod quality makes it look like darker brownish color but in person it's light brown. The rest of my sand bed is white so don't know why it's just this spot and what it might be.