Green Chromis dying mysteriously (pic)

[Truckie]

On 6/1 I added 6 Green Chromis (from Petco if that matters. Usually I go to an actual fish store) to my 55 gallon aquarium. They all appeared healthy - they were very active swimmers, schooled with my 2 clownfish, and loved to eat.

Two days after I added them they began to die off one by one. The first one disappeared without any indication of a problem. The next one began to show discoloration of his scales with light color patches and began to separate from the school and swim near the surface. One of them had a red mark on his side (I couldn't tell if this was from illness or an injury). All of them would separate one by one, show similar symptoms suddenly, and then die within a day or so.

A week later I am left with one chromi, and now he is showing similar signs as the others did. Last night I noticed that one of my clowns appeared as if he may have been picking on him after we turned the lights out. He rammed him as the chromi was trying to hang out at the surface of the water. I can't confirm if this was an isolated incident or not because it's the first time that I saw it, but my wife said she thought the clowns were picking on him earlier in the evening as well.

Should I be worried that an illness killed them that could affect my clowns as well? The clowns are happy as can be and haven't shown any indication of any sort of sickness.
Or is it possible that my clowns were killing them off one by one while I wasn't looking?
Either way I want to get this figured out before I added any other livestock to my tank.

Tank info:
55 Gallons
60+ lbs of rock and sand
2 powerheads totaling 2000 gph
Small hang on back filter with filter sponges

Ammonia: 0 | Nitrite: 0 | Nitrate: 0 | pH 8.2 | sg 1.021 | Temp: 76*F
10 gal. water change done weekly

Thanks for your help! Pic attached of remaining chromi with discoloration visible.

 

[SarahSmile]

This has happened to me as well, almost identically. Twice I've tried to add a school of 3 chromis, just to have them slowly change, lose color in patches, disappear, die off. Try doing a search here, a lot of us have had problems like this with them. There are some theories, they are caught using cyanide, so they are fine for a while but weaken and die, and the other theory is that they pick on each other til one is left, that is why some call them the "highlander" fish.
I have an awesome fish store here in Akron, two turtles, and I talked to one of the guys there forever about chromis, cause I can't keep them alive. He said they'll get in 15 of them and a week later, most will have died off and they will only have 2 or 3 left. He said they are weak and cheap. His words, not mine.

 

[Truckie]

This has happened to me as well, almost identically. Twice I've tried to add a school of 3 chromis, just to have them slowly change, lose color in patches, disappear, die off. Try doing a search here, a lot of us have had problems like this with them. There are some theories, they are caught using cyanide, so they are fine for a while but weaken and die, and the other theory is that they pick on each other til one is left, that is why some call them the "highlander" fish.
I have an awesome fish store here in Akron, two turtles, and I talked to one of the guys there forever about chromis, cause I can't keep them alive. He said they'll get in 15 of them and a week later, most will have died off and they will only have 2 or 3 left. He said they are weak and cheap. His words, not mine.

I searched a bunch of the forums and found similar stories to what you mentioned.
After last night I just wanted to make sure there wouldn't be any issues with my clowns being aggressive towards other fish. I'm guessing what I saw was just their clumsy swimming and not so much aggression towards the lone survivor.

The weird thing is, just like the other, he was acting sick last night and today he seems perfectly healthy.