what can i use to treat this? rin rot? thats reef SAFE

i have a bengai that has part of his fin receding, i noticed part of his tail fin (bottom part of fork at the end) was missing a piece, at the time it looked like a clean cut, now it is receding, not sure whats goind on but i dont want to loose this guy. i dont have a QT and cant set one up right now. i have corals (softies) and some cleaner shrimp. is there anything i can use that is REEF safe that would be affective? the only thing added recently was some xenia.

 

You Could try some melafix, have not used it myself but my friend has used it in his reef system with no ill effects with good result, its aloe vera extract i think, just an idea...