I've just been on the Waterlife website and you are using the correct treatment- Protozin. It says to do a third of a tank water change before using it.
As has been mentioned, i would increase the temperature a tad and add salt a teaspoon per gallon i think is right.
make sure you vacuum out the gravel at some point in case there is a lot of muck trapped there, you might want to wait 'till after the medicine is meant to have worked, but if you are going to do a repeat dose, do it before you do that.
don't feed for a few days, until he is looking more perky.
don't clean the filter under tap water obviously, i can't remember if you have carbon, but if you do, take that out to prevent all the medicine being sucked up.
make sure there is plenty of aeration going on, put an air pump in if possible, but make sure he isn't going to be blown about!
GOOD LUCK!