Fin Miscoloring And Part Missing

[ltb11uk]

My bala shark has lost the black line on his fin for some reason and a closer look shows sort of bite marks on the points of his rear fin.
He is in the tank with a red tailed who i have seen chasing but not catching it

Is it the red tail or fin rot?

 

[WILDER]

Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

Red tailed sharks are very aggressive so if he picking on the bala shark you will need to rehome one of them.
How many bala sharks do you have, as they shouldn't be kept in a less than a group of 3 as there shoaling fish.