The idea that fish only grow to the size of tank they are put in is a MYTH! What really happens is the fish's growth is stunted due to the small space. Although the fishes' skeletal growth is stunted, it's internal organs will continue to grow, and will eventually burst due to the stunted growth of the skeleton. This is extremely unhealthy and all fish kept in tanks too small for them die prematurely of organ failure. Also, NEVER listen to pet store employees. They do not care for the well being of your fish, and will sell you more fish than you can keep so that they make money, not only at that moment, but they make money when your fish die from stunted growth or other issues and you have to replace that fish. Please, for the sake of your fish, get a bigger tank or rehome the bala shark with someone who has 55 or more gallons to keep it in. I really hope you take my advice.