General Information: When fish show signs of illness but don't have any symptoms, sometimes they are diagnosed with depression. Of course this is a vague and often unhelpful diagnosis, but if you feel that this might be the case with your betta, it may be time to examine his conditions and environment to see if you can make a positive change. Prevent depression by keeping interesting things in your betta's home and water quality good. Don't overfeed.
Symptoms: betta sits in one place for long periods of time, clamped fins, may not eat, might not respond to movement, loss of color and personality, may not flare. Females may display horizontal stripes, a sign of stress.
Treatment: Many depressed bettas are in too small a home. If your betta has been in a half-gallon or one gallon bowl for a long time, it might be good to upgrade to a large bowl, or even better, a tank. Sometimes depression can come from overfeeding, and hence, lethargic behavior due to obesity. Make sure you're only feeding 2-3 pellets a day (go back to the Food section for more information). Also make sure that your water quality is always pristine, especially if you keep your betta in a bowl. Go back to the Cleaning section if you need to review how often to clean.
Probably the biggest and easiest way to help with betta depression is to add interest to your bowl (no, not for you, for him!). What does your betta do all day? Happier bettas have things in their tanks to "play" with: mine all of hiding places and plants to explore. Ornaments which are tunnels are great for bettas. If you do have these items and are still fighting betta depression, try rearranging the ornaments or finding some different ones, or try switching bowls or moving the bowl to a different place.
Does your betta get "flaring" exercise time every day, either from seeing another betta or in a mirror? If you have another betta, place the betta bowls next to each other for 15-20 minutes, maybe twice a day, so that they can make faces at each other. If you don't have another betta, you can use a mirror. In both of these cases, make sure that the bowl or mirror are very close to the other bowl; bettas are near-sighted. I haven't tried it yet, but I've also discovered this floating betta exercise mirror from ZooMed which makes the mirror method easier.
You may also try varying the diet a bit. Sometimes a boring diet can cause depression. Try bloodworms as treats and see how that goes over with Mr. Betta. I'm also looking at this three-treat set, also from ZooMed, which will add plenty more treats to your fish's repertoire.
Remember to keep aquarium salt in the water, maybe at half dosage if it tends to cloud your water.