Molly/ˈmɑːli/ Definition 1 of 3Pro Tip 1/3Aquarium Hobby Common in freshwater aquariums, used for home tanks. I'm thinking of adding a molly to my collection of tropical fish. Pro Tip 2/3Color Varieties Mollies come in various colors and patterns, often named accordingly. The black molly has a stunning velvet color that stands out. Pro Tip 3/3Live-bearing Fish Mollies give birth to live young, unlike many fish species. My molly fish just had babies, there are tiny fry swimming around.