Losing starfish

[majkm06]

About two days ago, my huge green spiny starfish (he had a 16" armspan, central disc 3 in across) had his tenticle sticking out of his hiding spot. He usually does this when he's hungry, so I then feed him big krill. He'll wrap his tenticle around it and pull it in where he eats it. I tried to feed him this day, though, and he wouldn't grab onto it. I tried for a while, then figured he wasn't hungry or my husband had already fed him earlier in the day. Didn't think much of it.
The next day (2/13), he was out on top of the rocks during the daytime. This is very strange and out of the ordinary- I've only ever seen him at night when the lights are off. I reached in my tong and tried to feed him again. He didn't do anything, but did try to get away from me. He didn't seem to want to be messed with/fed/anything.
Then yesterday (2/14), he was still out on the rocks. This time when I tried to feed him, he hardly moved. His condition rapidly declined throughout the day, until I found him at 1am on 2/15 dead. Without sharing too many of the disguisting details, it looked like his center disk had exploded. It was very strange.
I have two other starfish in addition to him. I QT'ed both of them. One of them is looking funny too, but he has been acting funny for the past several months. We call him our "senior citizen" starfish. He keeps getting flipped over on his back and isn't able to put himself back up. Once we flip him back over, he's fine and carries on. I figured it was old age as the three of them have been in my well-established 75 gal. tank for quite a while (years). He has never appeared sick or anything, just keeps getting flipped the wrong way.
The other starfish appears alright, but he's been in the same place for about a day. That's kind of strange (but only somewhat) for him. Sometimes he stays somewhere for a day then moves. May be nothing out of the ordinary yet, but anyways I wanted to note it.
So I did a water test to try to determine a cause of death on the green starfish. Here are my results:
pH 8.0
Ammonia 0.5 ppm
Nitrite 0ppm
Nitrate 10ppm
None of that seems off the chart bad. I'm not haapy about the ammonia or nitrate, but I wouldn't think that'd be enough to kill him. What do you think?
Tank has been established for 4 years. He was one of the first inhabitants once tank was established enough for a starfish. The other two were added over the years.

 

[beth]

You have ammonia which is bad news. Why are you experiencing ammonia. How long have you had this star?

 

[flower]

So you have had this starfish for four years, and the tank is 4 years old. Was it a large star when you first got it?
I assume the small ammonia spike is from the uneaten krill. Maybe he just died from old age, if nohing else in the tank is in distress.

 

[btldreef]

The ammonia is not a good thing, especially to starfish who really don't like it more so than other critters.
The ammonia could just be from the starfish dying, but that's a big spike for just a starfish dying, especially if you removed it soon after it died, which it sounds like you did.
I'd do a large water change, ASAP.
The starfish could have very well just died of old age, but if the ammonia was present before he died, I'd say that's what did it and it will certainly kill the others if you don't remove it.