I have a 200-gallon mixed reef tank. multiple Tangs.(Yellow, Black, Purple, Hippo, Gem). All are fat and eating well. had all five over five years. Over the last two years, the gem tangs fins have just slowly deteriorated. Now it is so bad she only has one pectoral fin and a partial tail fin. The dorsal fin is gone. She eats and otherwise seems fine. She is the most aggressive tang when feeding.