First setting up a QT tank is a great idea. There are many types of illness this may be, it may be columnarus, fungus or an injury that is getting infected. A picture would be helpful, also answering the following questions will help.
1~What type of fish is afflicted? In addition, please describe what is wrong with the fish to the best of your ability (i.e. cotton like growth, bloated, etc.).
2~What are your tank parameters (ammonia, nitrites, nitrates, temp, pH)? Please give exact values.
3~ How large is the tank? How long has the tank been set up?
4~What type of filtration are you using? Please give the name and number (i.e. Fluval 304) and amount of gph
5~How many fish are in the tank? What kinds of fish are they and what are their current sizes?
6~When is the last time you did a water change and vacuum the gravel? How often do you do this? How much water do you remove at a time?
7~How long have you had the fish? If the fish is new, how did you acclimate it/them?
8~Have you added anything new to the tank--decor, new dechlorinator, new substrate, etc.?
9~What kind of food have you been feeding your fish, have you changed their diet recently?