So I saw the white belly of a fish in my tank a couple days ago - assuming it was dead, I got my net to pull him out. But, he wasn't dead yet, even though he was floating abnormally sideways. Yesterday night I checked again, and now he seems caught in a perpetual left turn - i.e. his whole body is bent sideways. The below picture doesn't quite capture how bent he was, but gives you an idea. (He remains even more bent than the picture when released into the water.) What could cause this? The other fish were acting normally. I left my net partially submerged in the tank with him inside the net in order to partially isolate him from the other bluegill.
Air temp = 60-32 deg.
Water temp = 52 deg. last night, dropping slowly from 62 deg last week. I will use a heater to keep water temp above 45 deg.
PH = 7.2
Ammonia = 0 to 0.2 (my wife says 0.2, but I think less)
Nitrite = 0
I have been feeding the fish once every few days but only if the water temp is at least 55 deg. Thanks for any advice you can offer.