Before you add anything to your tank you need to read about the cycling.
Here is a link to fishless cycling using ammonia.
I am currently doing the above with my 50gallon tank.
The idea is that fish produce waste, wasted food and fish pooh which adds ammonia into your tank. By cycling your tank this creates a beneficial bacteria colony that deal with the ammonia by converting into less harmful chemicals.
Ammonia into Nitrite into Nitrate.
Using a testing kit such as the API FRESHWATER MASTER TEST KIT you can test for all the chemicals and their quantities in your water.
Once your tank is cycled you can start adding fish. Unless you plan on doing a fish-in cycle. But from your post it doesn't sound like you know about the cycling process.
Next comes your choice of fish.
The Bala shark through research of my own needs a very big tank. I was looking for my own tank but at 50g I only just have a big enough tank. Also Bala sharks aka silver sharks shouldn't be in pairs as they are very territorial. They dislike any fish with a remotely similar shape and they can grow up to 45cm in size
An alternative to a bala shark is another fish called a flying fox.
I suggest you read a lot more information before stocking your tank with fish that are probably going to die.
I am a newbie myself.