Mutilated Brittle Star

[Des]

I came home this afternoon to find this guy out of his hiding place and being picked at by other fish.
Tank is 5 months running, 120g
Animals include 2 Angel fish (coral beauty & lemon peel), 2 clowns, yellow tail damsel, lawnmower blenny, 3 red tip hermit crabs, and may have a couple others.
In the last week I’ve noticed my watchman goby hasn’t poked his head out, and I think it may be gone. A Royal gramma that I added the Saturday before last disappeared almost as soon as it was added to the tank. I also put an emerald crab in 3 months ago that I haven’t seen in a couple weeks, but that’s typical, it’sa good hider.
Ive had the star fish for 2 and a half months and fed him bits of shrimp by hand twice a week, but some feedings were missed when I couldn’t locate him.

Ph 7.4
Ammonia 0 to 2.5
Nitrite 0 to ?
Nitrate 5 to 10

I’m pretty sure the start fish is toast, but is there any way to save it? Could it be responsible for the other disappearances? And what on earth could have happened to it? I also see my nitrate is a bit high and it’s been stable at 0 to 5 for a few months now. Am I over feeding? Sorry for all the questions, I appreciate any feedback!

 

[blaxsun]

Starfish is definitely toast. Looks like a bacterial infection on one of the legs that spread to the center. Unfortunately there's not much you can do about this when it happens. Sometimes they lose a leg and it clears up, but if it makes its way to the center the starfish is doomed.

 

[dvgyfresh]

Nitrate is ok , I run my tank a bit higher. The thing that looks off is the PH it should be at a min of 7.8, I would look to correcting that , is there a top on the tank?