He needs to be put on a diet not fasted. That means to feed him properly every day and not just fast him a couple days a week and let him pig it out the rest of the week. Overfeeding is the main cause of betta deaths once they are given good homes. It is basically killing them with kindness. Bettas have no self discipline when it comes to food in a tank and as long as the food is there it is going to be eaten. My thoughts are to do the following:

Feed the betta on one end of the tank and give him one piece of food at a time rather than dumping food in at a large amount. The idea that they should eat all they can eat in a couple of minutes is ludicrous. They will overstuff themselves to the max this way. They should have a few pieces of food total and remember that the stomach of that fish is the size of its eye so it does not take a pinch or two to fill it. It only needs one meal in a day and many do quite well being fed every other day actually but since they get used to eating every day the notion of feeding them that way is fine but SMALL amounts are absolutely necessary. Swelling behind the pectoral fins and/or ventrals is usually a sign of constipation or overfeeding or possible beginnings of swim bladder disorder which if allowed to progress will be the end of your betta eventually. Bettas start to lose color and pale out and then become unable to swim or want to lay on the bottom of the tank and become less and less active and soon will be unactive totally. I am not joking either this is a very serious condition.

When the betta is having a problem the answer is not to restrict his movement so much as to restrict his eating. Probably if you are determined to keep him in this tank the answer is a tank divider not a box to limit his movement and cause a severe swimming restriction. That would compound the problem as he needs to swim to maintain his digestion process. He also needs proper tank temperature so what temperature do you maintain in the tank? It needs to be in the neighborhood of 80 degrees to help him clear his system of food that has built up in that fat little body.

If you need to restrict his movement just do it while you feed the other fish and do not be too over generous with them either. Obesity among all tropical fish is becoming rampant in tanks. Bettas are not the only ones suffering from it. Most tropicals do quite well with small meals one time a day and better than if overfed. There are problems that they have when overfed and most owners get to equating their own hunger with what they think the fish must need. NOT so. Don't be a permissive parent to your fish and do the thing that is best for them and cut back on the food to give them a better health. If you cannot restrict your betta any other way I would recommend seperating him from the others. They may be getting along but it is killing him to be with them if you allow him to have his way with the food.

The other option is to cut out all the different types of food and simply put in one type of food and let them all have at it and they will all (including the betta) get enough to keep them going. Put a small amount in and everyone will get some. They do not need to have a large meal but when you are feeding several types of foods it is like a four course meal everyone overeats. It will be fine this way too and also cut back on everyone's eating as they will have to share what is there. The betta can get by this way as he is demonstrating that he can and will eat the other fish's food so let him if he wants to but don't give him other foods too.

Rose