Halfmoon Black Mustard Betta Fish Females are stunning and colorful breeds of betta fish that are known for their vibrant blue coloration. These fish are relatively easy to care for, and they make great additions to any aquarium. If you are looking for a beautiful and unique betta fish to add to your collection, then a Halfmoon Black Mustard Betta Fish Female is a great option. These fish are relatively easy to care for, and they will thrive in a well-maintained aquarium.