I raise Bettas and am convinced that all males are just big drama queens. Females can be a bit grumpy, but they do well together in groups of five or more in community tanks. They just don't have the fashion sense the males are burdened with. And they do seem to have definite/distinct personalities. How recently did you buy the little guy? If it's been more than a week, I'd suspect there's something going on making him feel poor if he hasn't eaten, including not having eaten. If he refuses flakes or brine shrimp, try squeezing the innards of a frozen pea (after it has thawed) to float on the top of his water. Strange as it may sound, this often is accepted and it sort of clears out their little digestive systems.
You might consider moving him to a large bowl of his aquarium water and adding bottle spring water to make up the volume so he's got at least a gallon to swim around in. I can assure you, its more space than he's had to live in prior to you putting him in a new Betta tank. Once he's alone (and the water is the right temperature (78-80 degrees F), he may be more inclined to eat small bits of food. This way you'll be able to keep a better eye on him, check his poop and his body for any external parasites. If he doesn't improve, you'll also have him contained for treatment by adding about one teaspoon of canning or pickling salt per gallon to the water which helps the them recover from the stress caused by disease. The salt works by reducing osmotic pressure, enabling the fish's own immune system to fight back. <b>Note: do not use iodized table salt!</b> Not having observes ich or other parasites or fungus, that's the best advice I can know of. Hope the little guy recovers. Good luck.
