the most common cause is poor water quality encouraging protozoan infections that infect the fish. Doing a 50% water change and gravel clean each day for a week and see how the fish is after that. Also check the filter and clean it if it hasn't been done in the last month.
The tanks to small for an oscar and bala shark.
Plus bala sharks shouldn't be kept in groups of less than 3.
How the shark doing now.
I would do a water change for now till we know your water stats.