It's called "old tank syndrome"- when tanks deteriorate slowly, fish are pretty adaptable. They can become so acclimated to poor water quality that they actually struggle when the water is changed out with cleaner water- just because the change in parameters is so dramatic.
My advice- start doing a series of daily 10% water changes. After 5 days or so, if the water quality is still an issue, bump that up to 20% daily. Once you've got the ammonia and nitrites stable at 0ppm, then I'd medicate the tank.
Address the water quality first, though- that's the best medicine you can give the fish.
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