I have a 20 gallon freshwater tank that’s a few months old, maybe 2/3. It’s been cycled and my fish are doing great. I have some live plants, 6 corydoras, 2 snails(one nerite and one mystery), 5 Raspbora Hets, 1 clown pleco and frank the Betta. They all do great and are very active including the snails. My issue is that my tan sand has developed a black pocket. I haven’t disturbed it until I know what it is. In the photo it’s in the center close to the black bar at the bottom of the aquarium.