Angelfish dying

[Watcher74]

I have a minor problem in my tank I was hoping someone could clue me in on.
I have a 20 gallon tank with 7 neon tetras, 2 small(1"-1.5") angelfish, and a pearl gourami. I have it filtered with an electric filter with mechanical/chemical/biological filtering. I keep the temp steady at 79 degrees. Every week I do a partial water change of 25% with treated water that has been warmed to the same temp. My ph is 7.2, hardness of 6, no ammonia or nitrite readings.
But my smaller angelfish looks like he's on his last gill. He can hardly stay vertical, floating around in a rotating manner. He tried to eat a little but has no balance. I believe he'll be dead by morning. What I'm worried about is what caused it. None of the other fish show any signs of distress or disease.
Is this most likely an isolated incidence or is this the precursor to further die offs? If anyone has any suggestions on what may be wrong I would be very appreciative.

Watcher74

 

[LMOUTHBASS]

baby angel fish are very sensitive thats exactly how they swim before they pass on - i'm sorry - coulve been stress from anything - coulve just been a weak fish - if you got him at a chain store it was prob just a weak fish - petco angels are always very weak same for other chain stores i will only buy my angels from a local small lfs from now on after having similar experiences several times -
this type of fish IMO just isn't made to be shipped in huge numbers etc they just can't handle it well when their young

 

[ewok]

with angels, i have always read that you need the right ratio. either 1 or 3 or more, having a pair unless they are "a pair" is the wrong thing to do. they are pretty aggressive with others of their kind, so you need to spread that aggression. having 3 or more ensures that (hopefully) no one fish can be picked on all the time.

just some thoughts for the future........