I can't say I know whats wrong with your fish, but I would treat him with a medication that treats a broad range of external parasites. I say external because you say your fish is twitching? Have you seen him rub against anything or stuff like that...
When you say twitch, how do you mean? His nose twitching, whole body twitching, just the fins twitching? I do know that none of my bettas twitch (neither female nor male). Keep us posted.
well- i have no idea what happened. i did a routine water change- same as every single time. for some reason, all three of my male bettas got really sick. one started twitching- like his whole body would jerk, another one got a big black thing on his head- it looked like scales were turning black one by one. it grew as i was watching. and the third one didn't show any signs at all, but the second two died. the twitching one i dont think will make it either. he looks horrible right now. they were fine before the water change, but i honestly did it the exact way i've BEEN doing it every since i've had fish as a 5 year old. this has never happened before. sadly enough- the one that wasn't showing any signs of sickness and died was the one that i saved from ich last month. he looked BEAUTIFUL for a while.
I am sorry to hear about your fish. My betta fish died recently too. He just stressed out one evening. He actually "walked" on his back fin on top of the water, from one side of the tank to the other, then passed away and fell to the bottom of the tank. Maybe my betta and your bettas will be friends in fish heaven. Good luck with your next fish.
I know this may seem dumb, but you've been using a water conditioner that removes chlorine and chloramine during water changes right? It always strikes me as odd when a fish dies after a seemingly harmless water change.
Was the water company perhaps "flushing" the system? Occasionally they do this, and basically super-chlorinate the water. There's usually a notice about it in a local paper or something.