Popeye is not a desiese in itself, but could be an indicator of poor water qaulity. Usually high ammonia, nitrates, or nitrites. Could also be a bacterial infections. There's a whole list of things that could cause it, even stress is said to cause it sometimes, and tankmates can sometimes try to claim an eye and make it bulge too. I'd say you are doing fine on water changes, but if saturday is the only day you can do it I would personally do 50% until the eye gets better. Too bad that's the only time you have to do it. When my fish are sick I do daily water changes of %25-35%. Seems like it just helps them heal faster. Test your parameters, and if anything is off you might want to take a look at your filtration system to see if your biofilter can handle the amount of fish in your tank.