betta may be dying

Question
Hi,
I am a relatively new betta owner and have 3 male bettas. 2 of them I was keeping at work in a 2.5 gallon tank with a divider. The office is pretty cold & I'm pretty sure the water in their tank has been too cold, too. One of the fish became sick about a week ago. He seemed to be suffering from swim bladder disorder as his back side was bloated & he was floating to the top. I tried feeding him a frozen thawed pea & was having good success with it, but it didn't seem to help his sbd. I let him fast over the weekend & tried to feed him part of one Monday but he wouldn't eat & suddenly his Finns are beginning to look ragged. So...I have a betta that looks like he has sbd & fin rot but treatment does not seem to be working at all.  
I change their water at least once a week. The water is room temp & I treat the water prior to putting them in it. I feed them new life spectrum betta pellets with the occasional freeze dried blood worm. The other 2 are doing just fine. I have moved the sick one into a temp tank but I don't know what to do for him now. Any help would be appreciated!

Answer
Danni,
One male Betta should be housed in a tank no smaller than 2.5 gallons, where a 5 gallon is best. When they are in such small tanks, it is hard to not only keep them healthy, but happy as well. In the tank, there needs to be a heater that is keeping his water at 80 degrees at all times. The constant fluctuations are not good for them and will bring stress which then brings illness. When you feed the pea, make sure it is cooked and the outer shell is removed. He will not be able to digest the outer pea shell. Try it this way and let me know how he does with it. As for the fin rot, just add some dissolved aquarium salt to his water at the ratio of 1 teaspoon salt to 1 gallon of water. Once the fin rot is healed, you will have to remove the salt by doing 25% water changes every day for 5 days.