Lifeless Angelfish Not Eating

[Nyan]

I recently purchased 5 angelfish for my tank, and one was a little larger than the others. They're all still young though. The smaller 4 angelfish are doing really well, and are happy, healthy and inquisitive. The older one has acted strangely since the day I got it. It used to spend all day staring at itself in the glass in one of the back corners of the aquarium. Although, since I've gotten a light for my aquarium, it sits at the top of the water, near the back, staring up into the light.

I haven't actually seen it eat since I got it, about 3 weeks ago. It doesn't pay any attention to food. I've only ever seen it put some flake food in it's mouth twice, and both times it spat it straight back out and spent a minute acting as it was choking and/or trying to spit something else back up. It's never shown any interested in blood worms or brine shrimp either.

It constantly breathes heavily and looks like it's gasping. The levels in my tank are, and have been the whole time it's been in the tank, stable. Readings;

Ammonia - 0
Nitrite - 0
Nitrate - 10
PH - 7.6

I've been watching it closely since I got it, and no one is bullying it. The other fish treat it like it's not even there, and pay no attention whatsoever to it.

Is there something I can do for my fish?

 

[Nyan]

I've taken videos of my fish. The little white bits you can see floating the water is actually small air bubbles. When I added the light and the fish went up to the top like that, I freaked out, thinking that it wasn't getting enough air. So I've just been covering all my bases, although the other fish hasn't be bothered by anything at all.

Here is a link to a video of my healthy Angelfish, and then the other one;

And then here is a video of just him, gasping for air;

 

[Nyan]

It's now starting to swim funnily/weakly, flipping upside-down, on its side and resting in plants. This has only started in the last hour. Is there anything I can do to save/help it?

 

[Nyan]

My little Angelfish has passed away. ):

 

[anteatergoanna]

I am so sorry to hear that.

 

[Mama Karoti]

I'm sorry. Sometimes it is difficult to figure out what is wrong when there are not many clues.