Pls tell your brother not to go over the balas sharks. I wouldn't recommend balas for a 75 gallons. They are very skittish and will grow to 30 cm max. Believe me..this is stressing for the angelfish who will not like fast movements of a 30 cm bala shark.
Balas are shoalers and 6 will often be the minimum just to make them feel more secure. The more they are scared, the more likely they'll jump out of the tank or worse, injure themselves from their mad dash.
6 Balas need 150 gallons as the minimum, not 75.