Make sure to acclimate slowly (it will lower your ph) and make sure it's pure 100% magnesium sulfate per the ingredients label. No dyes or perfumes of any kind. It does need to be predissolved and added slowly over an hour or more.
I've never needed more than 1/2tsp per gallon but people on these forums like to use a lot more.
What color is your betta's poop? What the ammonia, nitrite, and nitrate levels of the water? More info will make sure we're not missing anything: http://www.bettafish.com/showthread.php?t=49233