Had a mortality - Help save his tank mate

[Lofus]

I'm not sure what happened to this Green Chromis. He has been in the QT for two weeks and I noticed yesterday that he was swimming slightly off kilter. The QT is 10 gal with a bio wheel filter. The water specs are:
Temp 26C (in the past two days it has ranged from 28 to 25)
Ammonia 0
Nitrite 0
Nitrate 0
Sp G 1.023 (was up to 1.027 a couple of days ago but I've been adding freshwater to bring it down)

I do 20% water changes every other day with water from my main tank (same water specs but 27C).

I thought he might have had HLLE because I could make out a black line from his head along his side to the tail. I noticed the line two days ago. He had not been eating as well as the other chromis in the tank.

The LFS told me that is shuch a small QT I should only feed them once every three days and only so much as they could eat in 3 mintues.

These are/were my first fish and I would sure like to keep this last one alive.

I did notice that they looked a little blotchy but after I turned the canopy light on thier colour came back so I figured it was nothing.

I posted a post mortum picture on www.reefs.org too.