Bala sharks grow up to 25cm when fully grown so it is wise to buy a large enough tank to house one.
If you have anything less than 200/300L don't even bother thinking about a bala shark. I was also surprised at how fast it grew.
When you buy them in store they are about 3/4cm in length, within 9 months or so and it was easily 10/11cm.
Eventually I could see that this bala shark had out grown it's tank and so that's why I ended up returning it to the shop I bought it from.