Okay, well first thing, I would definitely begin using garlic and vitamins. These will give your fish a stronger immune system and at least help the fish get nutrients (since it is not really getting much from just eating one type of food). Good vitamin supplements include Zoe, Selcon, and VitaChem. Kent Marine makes a galric additive called Garlic Extreme, or you could use fresh garlic that you would buy from the grocery store.
Also, you really need to know an exact number for your salinity. Within a range is not a good thing because salinity fluctuations can be very stressful for your fish. Most hydrometers put the safe range between 1.020 and 1.025. If your salinity is within range and a different number every week, this could be stressing your fish out greatly.
Also, alkalinity can be tested with an alkalinity or kH test kit. Alk/kH is very important because it tells you how stable your pH is. Your pH may be 8.1, but if you have a low alk, it could be dropping to 7.6 by the end of the day.
I would fix whatever is stressing your fish out. That will remove the popeye. I would begin doing more frequent, larger water changes, get the garlic and a few vitamin supplements, and pick up an alk test kit. You will only fix the eye problem when you fix the fish's stress.
How often are you gravel vacuuming/doing a water change (what percentage) and how often are you feeding the fish?