Bala sharks are incredibly peaceful fish, usually, but are schooling fish and won't be happy alone, so perhaps yours is showing an unusual aggression (Bala sharks also get fairly large, up to 12" I believe, and aren't good choices for a 40g tank). I don't believe any of your other fish are overly aggressive, and if your water parameters test good, you may want to look for other causes: parasites, bacterial or fungal infections, etc. Did you also test for ammonia? Nitrites and nitrates can test good, even if there are unacceptable ammonia levels in the tank.
150g long S Amer cichlid; 90g C Amer cichlid; 90g Malawi cichlid; 40g Tanganyika cichlid; 29g "Shellie" cichlid; several smaller tanks. 6,500g koi/native catfish pond; several smaller ponds