Tail Fin Loss

[Biocube32er]

Hey guys I have a Royal Gramma and I notice that his tail fin is damaged pretty bad. I was getting worst to the point he has almost no tail. I don't see any aggression at all towards it. I sit right next to my tank and I have an eye on it all day pretty much. Could something be happening a night? No of my other fish seem effected.

Thanks

 

[Reefer40b]

Sounds like some tail rot you can use API fungus cure for that.