Hi kaynmil and welcome to the forum. dbrooke asked me to take a look at your thread. So you have a betta with popeye?
The first thing you'll want to do when treating popeye is add some epsom salt, 2 tsp per gallon, to the tank. This will help reduce swelling and fluid buildup in the eye. If the epsom salt alone doesn't reduce the swelling within a few days then the best medicine to use is actually API Erythromycin or Maracyn I. Many bacterial eye infections are gram positive bacteria and Maracyn II won't treat them. Try to use just the epsom salt for a few days before attempting a med.
If you can find Indian Almond leaves or naturally dried and clean oak leaves, use those as a beneficial source of tannins. That will help immensely as well.
It's just fine to treat in an unfiltered tank. In fact, in some cases, it's best.