A sick Lionfish

[dawn m. gray]

Good day any help would be greatfully appreciated. I have had a Saltwater tank for over two years now starting out with a 55 gallon doscile tank until my sons purchased a lionfish for me as a gift. He was approx.4" at that time that was in Dec he is now close to 11" and in March moved him and all of his buddies into a 155 gallon. Water quility has been good, eating has been good. in the past 48 hours he stoped eating and is sitting at the bottom of the tank. I was able to get him to move and noticed what looks like a tumor just behind his front right gill and a dark maroon like color moving up from the tumor to the side of his fins. Not sure what is going on, have seen nothing in my marine books that remotly looks like this and how to treat or if I should treat. He is not eating, he is reponsive to myself and son. I have in this tank 3-top hatsl, 1-red tooth

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trigger, 1-2ft snowflake eel, 1-hippo tang ( the tang was just recently added in the past week. Hermit crabs and excellant alege growing lot of purple.