In my experience Bala sharks are one of the hardiest fish out there, but saying that, very young juveniles such as yours, as with any juvenile fish, can be susceptible to stress etc.
And another thing that I have learnt from bitter experience, and not many will agree, is that quarantining such juvenile fish can end in disaster. Tiny juvenile clown loach were always problematic for me. Then when I stopped quarantining them they started surviving, go figure, lol.
I realise now that my methods of quarantining were not ideal. I'd set a tank up especially for new arrivals, with seeded media etc. Good to go, right? Not necessarily. I believe quarantine tanks need to be just as mature as well aged set ups to give new additions, especially tiny juvies, the best shot.
My abject failure with clown loach taught me a lot.
I don't know what your quarantine procedures are, I'm just offering my perspective. My original group of Bala's were 2" when I got them. No quarantine, no problems whatsoever, the biggest is now about 11" and seven years old.
Or it could be something as innocuous as a "bad batch" with weak genes, who knows.
