RubyMutt is right about the water, but in my case we have well water and the PH right out of the tap is high. I don't know where you live but just in case you might have well water.....check the PH. Some well waters have low PH some high like mine.

Also did you happen to have a plant in with the Betta, I had problems with one Betta when I put the plant in with him. Within a day he died. I found out that you have to wash the plant really well, just in case there is any clems. on the plant. I washed it but I guess not well enough.

Sense I got rid of the plant, my Betta has lived for a year now.

One more thing, did you clean the bowl with soap, if any soap was left on the bowl the Betta wont make it. Belive it or not, Bleach is better to clean fish bowls with if you have to clean it. Rinse it well, but even if there's a little bleach left, they are more likely to survive it then soap.