Hi
What size is your tank and how long has it been running for?
How long have you had the fish?
Whre did you get it from?
Are there any other fish in that tank?
what are your water stats?
I know it seems like a lot of questions but they are very important.
Sounds like swim bladder then, is the fish bloated, can he maintain his balance in the water, does it look like his tail weighing him down.
Does he swim upside down.
I would try some shelled peas on him, cook peas for a few minutes, let cool down, pop out of shell, mush between fingers and add to the tank.
Don't feed flake food for a few days.
Get some frozen foods daphnia is very good for digesting the fish food.
Not got a good batch of them, no don't add the bloodworms will make things worse.
To be honest if he's swimming upside down he's not going to make it sorry, i would put him in the net then.
R.I.P. to the one that died.
I wouldn't feed them for aday, and no flakes they need flushing throw with peas and daphnia.
As long as the fish can't peck at him they should be fine.
could be internal parasites or it could be stressed??