Anthias tail going missing??

[metalhead789]

My Bartlett anthias' tail has been getting worse and worse over the last week. Slowing its receding and is somewhat fraying. It doesn't look like fin rot which is what strikes me a odd because I have to clue what it could be. The back fin is the only one that is affected. He's in a 75 gallon tank with some coral, a small yellow tang, a seagrass file fish, a lawnmower blenny, a sand sifting goby and a few crabs. Salinity is 1.0248, ammonia 0, nitrate 0, phosphate 0.025 or less, nitrite 0, calcium 460 - 480. Any clues as to what it could be?

 

[metalhead789]

My Bartlett anthias' tail has been getting worse and worse over the last week. Slowing its receding and is somewhat fraying. It doesn't look like fin rot which is what strikes me a odd because I have to clue what it could be. The back fin is the only one that is affected. He's in a 75 gallon tank with some coral, a small yellow tang, a seagrass file fish, a lawnmower blenny, a sand sifting goby and a few crabs. Salinity is 1.0248, ammonia 0, nitrate 0, phosphate 0.025 or less, nitrite 0, calcium 460 - 480. Any clues as to what it could be?

Forgot the pictures!!!