Please help. Tang is getting nipped

[ShuMurph]

I have five fish in my 54 gallon tank. Tang, dotty back, 2 clowns and a damsel. Fish only no coral

At first I thought it had fin rot so treated the tank for 7 days then did a water change. Fins look they are getting worse. Is there anything I can do? Heard that the dotty back could be the culprit?

 

[ReefDudes]

Dotty backs can be nasty so its a good suspect.. I would just hang out and watch the tank in the evening to try and spot the aggressor