new to the forum, have a question already! 37 g tank. 5 small clown loach, 3 neons, 5 glow. Question has to do with Sand. Have had the sand since setting up, over a year ago. Seeing dark discoloration in the corners and a small amount through out the sand. I do stir for gas. Not certain what this can be and/or how to get rid of. Any thoughts?