Major breeders (I wouldn't call my self a major breeder since I work out of a humble fish room :P) prefer to use treated tap water.
It's basic to treat tap water and get it ready for your fish:
Just get a solution of Stress Coat. It's probably the best water conditioner for bettas. THen get the water in like a jug or whatever. Put the correct doseage of Stress Coat in the tap water and wait until the tap water is of room temperture (leave the light off). Then pour the water in your tank.
You can later turn the light on and give your friend some warmth :)
You should look around for fish diseases and cures (www.bettatalk.com
) and see whats up with him :)