other causes for the popeye would include injury due to trauma, bacterial infection, and eye flukes (parasites), virus infections, vitamin deficiencies, osmotic problems in the water chemistry, systemic infections of any types.
This is why I suggested a strong treatment of Maracyn with the clean conditions. The spring water is okay to do without as long as you can treat the fish with the medication. Since we do not know that the problem is not caused by an infection or trauma that is now infected he really does need to be medicated.
Also what is he eating? It may be that he is not eating enough to keep himself nourished but he does appear to be healthy and nourished so I am really doubting that vitamin deficiency or malnourishment is the cause. I also really doubt the eye flukes being the cause unless you have other fish with the problem unless he was the only fish in the tank. They tend to spread like wildfire.
If your water testing is showing good parameters in ammonia, nitrites, and nitrates and you have not had any other sick fish with this problem then you definitely need to get this fish some Maracyn as he has a bacterial or viral problem that will not be simply wished away. He also will probably need daily water changes of some small part of the water for about a week or so to help the problem disappear more quickly. Even 25% of the water changed daily for a week will help him to improve more quickly. The carbon needs to be removed from the filter for the length of time you are medicating and a new media with carbon put in after the medication has cleared the problem. He should not need more than a week for the swelling to go down completely but the cloudiness may not go away for a while after that and your little guy should not go back into his main tank until he is totally back to normal.
It is not unusual for fish who have had this happen to be oversensitive to water quality problems in the future so don't be surprised if he will be the one you will have problems with should you miss a water change or have a problem with your tap water having higher than normal nitrates or anything in the future. Just keep the Maracyn on hand. I once had a betta like him and he had popeye about one time a year when the towns water had higher nitrates than usual. I could plan on it.
Rose
Since I do not know where you are from...the chief ingredient in Maracyn is erythromycin and any medication that you can find that contains that mainly will work. Since that is the main component of this medication I like to use it as it does not have a lot of other things in it to bother the filter. But if it is a problem any other med containing that will work.