You mean you actually put a bala in with your pirhanas? Im surpised it last that long.
A bala isnt a 'real' shark. Shark isnt maybe the right classification for it really. The common names for Balantiocheilus melanopterus derive from the fishes shape and not the predatory character associated with their larger marine cousins. Bala has no chance with pirhanas. Pirhanas have razor sharp teeth. Ive heard of people getting little pieces bite off of their hands too. A bala does not have teeth (I dont think anyways).
And addition to all that, your tank is too small for a bala and the pirhanas. Balas can grow up to 12". Due to it's size and shoaling nature, a long aquarium (>1.5m) is recommended. Red Pirhanas can grow to 11".
Where do you live? US? Canada?