He may have been infected with something, I'm not sure how to put it though. Never dealt with cloudy eye before, so.... yeah
IMO, 1 gallon is not
enough for a betta as a permanent home. Ammonia builds up too quickly to maintain a nice home. And physically it's not that good either. They have more trouble turning around and such, with their tails. And because of their fancy tails, they deserve more room to freely swim. So I prefer 5 gallons, but 2.5gal easily makes a better home. Just my 2 cents.
However, many breeders use homes smaller than 1 gallon, but that's becaause they understand how it works. They know about the diseases, the ammonia, the everything, so they're experienced enough to make it work. And usually the less than 1 gallon is not their home, either. The difference from a pet store is that the water is MUCH much cleaner. So much cleaner. Especially those ewith drip- ______ (I forgot the word...)
And 1 inch per 1 gal is ONLY a very, very, VERY basic rule of thumb for new aquarium owners. As soon as one gets more experienced, you'll see why it doesn't work that way entirely. (A 10 inch fish in a 10 gal? I don't think so!). So really it's based on their bioload. You could have a dozen shrimp in a 1 gal, because their bioload is ridiculous! It's so small, it'll work out fine. They're not big on space either, and can work with little space. With water changes, of course, since they're picky about water.