all you can do is keep your tank levels where they're supposed to be, and keep the temperature where it's supposed to be. Dirty water and flucuating tank levels will cause popeye and other bacteria. A tank that gets too hot will also lead to less oxygen and easier infections.
once a lion gets popeye, a 25-35% water change and a little good luck will save him. haven't heard of much luck with popeye medications and lions, but it's worth a shot in a QT tank. Use Maracyn-2 in the QT and if you catch it quick, this medicine should work.