Little Black moore dying. Please help

Hello
My poor little black moore is in very bad shape. He has curved his
small body and has sunk to the bottom of the tank lying on one side
and breathing very slowly like once every 5-10 seconds. Please help if
you have any suggestions. He is about 1 year old and ~ 1.5 inch long.
He is in a hospital tank now and I have added 0.5 teaspoons of salt
per gallon. There is an air filter and I am changing the water in the
morning and at night. (Actually I have two tanks so each tank's water
is aged 12 hours before he is moved into it). He was looking healthy
and really energetic last week but his sudden sickness started two
days ago. His condition is almost stable or maybe getting a little
worse very slowly. He had a series of bad luck. First when I added
some medicine for my other fish he thought it is food and ate a lot of
the medicine while it was still floating on the water surface. The med
was for rotten fin and it is called NITROFURA-G, see
http://petgeneral.safeshopper.com/46/440.htm?536 . Before that he may
not have been eating for a few days since it looked like the other
fish didn't let him eat. Unfortunately I had not noticed this. Any way
yesterday, when he was still in the original tank, I noticed that the
water filter had sucked him (almost) in. It seemed that he had lost
his energy and maybe while he was swiming in the vicinty of the filter
he was not able to fight the water succsion. I think he was in that
situation before I saved him. I am sure that has stressed him a lot as
well. Any way he has been in the hospital tank since then and my only
resolution right now is to keep the water quality superb and the rest
to his immune system take over. Please provide any suggestions that
you might have.

Thank you in advance.

Mazir