Good morning and WELCOME to Fish Lore.
Can you tell us your readings for
ammonia,
nitrite and
nitrate? It would be a great help.
Is it the Rainbow Shark that you are concerned about? If so, how long have you had it? When I introduced mine into my 155g I didn't even see it for 6 weeks. He was terrorized by my Bloody Parrots at first. It could be possible that your bala sharks are doing the same to yours??/
In my opinion you are a bit over stocked especially having 3 Balas (that have the potential to grow to 13") and a Rainbow Shark. Too chances are that the Rainbow Shark is eating and you just don't see it happening. If it's new, it will need time to adapt before he is free swimming with everyone else.
Best of luck and keep us posted. I'm sure you'll get some more responses as well.