Starfish

[Double J]

Just noticed my starfish not looking good when I was feeding the fish. He was out in the light and had hermit crabs around him and is missing a leg. Not sure if the picture shows it to well but his body has turned more white and there is a dark area in the center.
Any ideas what could be going on? He has been in the tank a year. Thanks

 

[Double J]

Better pic

 

[Tahoe61]

I would do a run down on water chemistry first, including Nitrates, SG...
Are you directly feed the Serpent Star?
Are there any larger or more aggressive fish in the system such as puffers....

 

[Double J]

Have been battling nitrates for awhile, around 25ppm. Ammonia and nitrite at 0 haven’t checked po4. Salinity 1.026, well it did drop last weekend due to ato overfilling, but is back down now. Fish are 2 ocellaris clowns, 2 Bangaii cardinals and 2 blue shappire damsels. No direct feedings either. Thanks

 

[Tahoe61]

That nitrate value would not concern me.
You should feed the Serpent, they are voracious eater. Extremely hardy inverts so I imagine it's not getting enough food and what food is left over from feeding the fish is quickly cleaned up by hermits and such.
Unfortunately the Serpent may be too far gone based on possible tissue erosion to the disk/body but I would still offer tiny pieces of shrimp, fish..squirt some frozen it's way.

GL