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Bala sharks, also knows as silver sharks, shark minnows and tricolor sharks, aren't sharks. They're large, tropical freshwater fish who grow up to 14 inches long. They resemble sharks slightly, thanks to their top dorsal fins, but bala sharks are bony fish. Determining gender is a challenge, but it's easier if you know the fish are the same age.
When you have two adult bala sharks of the same age, you can determine gender by comparing their sizes. Females tend to stop growing before males. Male bala sharks often are slightly longer and taller than female balas. You must look carefully to determine which fish is bigger; the size difference isn't dramatic.
Female bala sharks often have fuller, rounder bellies than males. Males are more streamlined in shape, although the difference might be subtle. It's more obvious when mating time approaches. When a female is ready to lay eggs, her belly looks rounder than normal. Males don't change bodily shape when it's time to mate.
With the right timing and a little luck, you might catch sight of your fish breeding. Watching the breeding process definitively tells you which fish are which gender. Females drop eggs along the bottom of the tank, then the males swim over the eggs and deliver sperm. Adequate filtration keeps the current moving enough to spread the sperm throughout the bottom of the tank to cover areas where eggs fell. If you miss the initial dropping of eggs and sperm, you're out of luck using this as a sexing tool; the fish don't stay near the fertilized eggs.
With some fish species, the males or the females care for the eggs and the young, helping determine adults' gender. That's not the case with bala sharks. After laying eggs and depositing sperm, bala sharks don't spend any time caring for the eggs. When the eggs hatch, the babies are on their own. Adult balas sometimes eat their own young rather than nurture them.