Is this fin rot on the dorsal fin? I found 2 of my Monos like this this afternoon. I have a 90 gallon marine reef tank that has been up and running since September. These 2 fish were added in mid December (along with 3 other monos and 2 clownfish)
I did a 10% water change yesterday and did a test this afternoon after discovering the fish like this. My results were:
ammonia -- 0ppm
Other additional info I can offer is last Wednesday I added a flame angel and a Blue Tang. Other tank inhabitants include a skunk basslet, a royal gramma, 2 skunk shrimp, 9 hermit crabs, 1 brittle star and 2 emerald crabs.