Can i put fungus cure (aquarium pharmaceuticals brand) in a aquarium with healthy fish?

I agree that complete water changes are VERY stressful for your fish and should be avoided. 40-50% every other day in emergencies are much more effective. To treat funguses I would use a combination of Primafix and Melafix (5 ml of each) for at least 2 weeks or a week after visible signs disappear. Both of these medications are naturally based so they are not nearly as hard on your fish as traditional medicines (that often stress fish and lead to secondary infections). I would also recommend adding 2 Tbsp of aquarium salt (not table salt or marine salt) per 10 gallons of tank water, dissolved in a cup of dechlorinated water first. This is also how to dose powdered medications to avoid having them consumed by your fish, by pre-dissolving the medication in a cup of water. Feed sparingly while they are sick as they will eat less and the food will rot and foul the water. Good luck.