I have a sick Imperial Angel!

[Fegenbush]

My Imperial Angel has a spot on it's right pectoral fin that started out small but now is the size of a pencil eraser! I will post pics to see if any one can tell me what it is and how to treat it. I just bought a ten gallon tank set for hospital.
My main tank is 75 gallons with live rock, Grouper, very large Lion Fish, and a 24 inch Snowflake Eel
My Ph is just right, my salinity is perfect, my ammonia is 0, my nitrites are 0, my nitrates are a little high. The Angel is eating great no problem there. I did start a new type of bio-enzyme water treatment system called "Aquabella" it is 100% organic and adds naturally occurring non-pathogenic micro-organisms that optimize the decomposition of waste water pollutants. Your not supposed to have to change the water for a year guaranteed, If any of you are familiar with this stuff let me know. I don't think that it has anything to do with this fish. the others seem to be getting along just fine. I also have a choclate chip star and two pencil urchins. they seem to be doing fine also the star has a great appetite.
Please Help!!!!!DSC00155.jpgDSC00157.jpgDSC00159.jpg