What wrong with my goldfish?

my goldfish yesterday was folating on its side in its tank, the water in the tank was very very smelly but was only cleaned a few days ago and was fine anyhow you couldnt even see through the water so i took the fish out and put it in fresh water which i added the water purifier etc to. the fish seemed to be getting better by making the effort to swim and was more active. i read online to add defrosted frozen peas (shelled) which i did but now its just floating not moving at all but its eyes are moving what should i do please serious answers only no nastyness as its still a living thing ( ive read some horrible things reguarding this )