Another angelfish on the brink of death

[ImBettaThanCool]

This has happened to me twice now. I have a group of small dollar sized angelfish in my 40g along with some other fish. I do water changes weekly and everything is fine. The thing is though, just a month ago, one of the angelfish started to open and close their mouths sort of quickly(not rapidly). That angelfish died the same day. I was not able to find the cause of sickness and death.
Now, I have that same issue. Yet another angelfish is lying motionless at the bottom of the tank, slowly gasping for air. I still have no idea what's wrong. Any ideas for saving him before its too late? I'll try to post a video
Here's video link
https://youtu.be/ty2aTsFFiCA

 

[V227]

You indicated that the water parameters are solid, and the Angel was acting and eating normally up to the point of the illness. You added that the the death occurred very quickly after the first noticable symptom.
If that's the case my guess would be a Bacteria.
If the Angel became lethargic, stopped eating, a week or so before death, then I'd guess, Intestinal parasites.
Regarding, bacteria; prevention is truly the best med. Pristine water. I have yet to read a success story on any forum, where antibiotics actually cured an infected fish. Some experts recommend aquarium salt and salt dips. I've not done that.
I'd recommend removing any sick fish (asap) at the first sign of illness. It might help matters to conduct a 40% - 50% WC. Sorry for the loss of the Angel.

 

[ImBettaThanCool]

Thanks. The angelfish did die. BTW he gradually stopped eating throughout the week until today where he just started showing those symptoms. I have already done a 50% wc and will continue to do 25% wc in the future.

 

[V227]

That sounds like intestinal parasites. Weakened fish are susceptible to all kinds of bad bugs that also inhabit the aquarium. This sort of thing happens to all of us at one time or another.
Just keep the tank ultra clean, things will level out.