If you see a brown or black, dirt-like substance collecting in your fish tank gravel or sand, most likely it is mulm (also known as detritus or debris). When organic material – like fish poop, leftover fish food, and plant leaves – decompose in the aquarium, it turns into mulm, which is very similar to the compost you use in gardens. Mulm may look dirty, but it's full of nutrients that are extremely beneficial for aquarium plants. Read our full article on mulm
to find out how to remove or hide it.