Hi and welcome to the forum
What fish are you treating?
Why are you treating the fish, what is wrong with it?
If you can post a picture of the fish it can help us identify the issue.
You never use medications unless you know what is wrong with the fish.
You always use a filter and increase aeration when using medications.
Melafix, Pimafix & Betafix can leave an oily film on the surface of the water, which can cause fish to suffocate. Make sure there is some surface turbulence to prevent this from happening.
Most fish health issues are caused by poor water quality and doing a 75% water change and gravel cleaning the substrate each day for a week or more will fix most problems.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
The following link has information about what to do if your fish gets sick. It's pretty long and boring but worth knowing. I recommend printing it out and reading it in bed to help fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/#post-3804819