Yellow Tail Damsel -- Odd behavior
So I have a 55 gallon tank. I believe it finished cycling two weeks ago. Right now all I have in there is one yellow tail damsel in the tank right now. I bought two but unfortunately the other one died. Now, after about 2 weeks of normal behavior without any real problems, my surviving damsel has been having a hard time swimming. My setup is kind of simply, I just have an ehim ecco filter right now, and no protein skimmer.
His coloration is okay, he seems to have lost a little bit of color on his belly, but other than that it is okay. There is no signs of redness on his gills, so I don't think its ammonia, especially since my ammonia is below .25 right now. I know that's not great, but I thought you would need more than that to kill a fish.
He kind of floats around, but he is using all of his fins and his tail. It's like he is, I dunno, drunk. He can't swim straight, and he keeps going vertical with his nose/face towards the gravel and his tail up in the air. He doesn't seem to be breathing heavily. It's almos like theres some sort of balance issue going on. He'll float around, trying to straighten out, but he'll just go with whatever current he's near.
Also, although he's been eating fine the last 2 weeks, yesterday he hid most of the day and didn't really eat much, if at all. I tried feeding him today, and he showed no interest.
Any help would be greatly appreciated.