It depends what you want to do.
If you NOT going to cycle your tank after you done with treatment ,then i would just do 100%. And you can take the filter out and do regular water changes after you done with treatment. Not sure why you need light in the tank.Don't keep light 24/7 though, it will stress him out. Is your room too dark? Rinse the filter every about 2 days while you treating him in the tank water.
If you going to cycle your tank and keep the filter in then do 50% daily.
I hope he will get better. Still not sure if he is biting his tail though. If you ever see ''u'' shape bites it can be tail biting. No discoloration, often has u shaped chanks missing would be tail biting.
When you finish with any treatment and hopefully he will get better make sure you always rinse/swish the filter media. Sometimes filter cartridges can gather a lot of gunk on them and that could cause ammonia problems. The link that i gave you and post #22 has the link on water changes for cycled and uncycled tanks. It from Oldfishlady so i am recommending to follow her instructions:)