What's wrong with my guppy/ tank?

[Lo Ki Gorgeous]

pictures and video of the fish?

if fish bloat up (get fat) suddenly, its an internal problem and usually an internal bacterial infection that can't be treated.

I didn't take any photos of them unfortunately
But if it's bacterial will the others be at risk?

 

[Colin_T]

It's possible the other fish are at risk but it could be a one off situation whereby the fish was just weaker and got sick.

Monitor the remaining fish and if any act unusual, post pictures and video of them straight away.

Check the water quality for ammonia, nitrite, nitrate and pH.
If there is a problem with the water quality, it will kill fish very quickly.

 

[FishkeeperDiscoWostit]

Hi guys...

So... I treated the tank with salt for three weeks and two days..
Then I gradually changed the water every day for two weeks.

Then I waited one week (still changing the water several times) and then I added some guppies my sister couldn't have anymore.
I was a bit worried but I had to try putting fish in the tank sooner or later so..

Now.. after a few days one of the male guppies seemed very lethargic and was only hanging out by the surface of the water with his head upwards and his tailfin hanging low underneath him. He was almost vertical, all the time.
I noticed his belly seemed very very bloated so I put him in a quarantine tank by himself and read that if he was compacted he could only eat peas.. so I gave him a little bit of pea. He ate very little..
The next day he was still vertical and lethargic so I read that if he had dropsy or bloat I should treat him with salt. 1 tsp for 20 litres...
The next day he seemed more perky and was swimming around more..
But the day after.. he was dead
I inspected him and he didn't have any excess mucous like the guppies who were sick before I treated the whole tank.
He only had that huge belly and vertical lethargic behavior.

Now today, I opened the lid to feed the fish and one of the females were floating dead in the surface. She was bent to the side.. and her belly seemed completely sunken in. So the opposite of the male guppy.. she didn't have any excess mucous either..
She didn't show any behavioral changes before her death like the male did either.

So....
What the heck is going on...? Did I not get rid of the disease from before?
Am I just unlucky and the male ate too much or gulped too much air by accident?
Is it because I'm using flake food? I crush it up pretty finely when I sprinkle it in though..

And what happened to the female? Her belly was sunken..

I feel like such a failure.. those poor fish...

I would love to get you guys' thoughts

Connorlindeman is right, don't feel bad, guppies can die very easily.