Please answer these questions: http://www.bettafish.com/showthread.php?t=49233
Are you using any type of water additives such as a water conditioner? Water, purified or not, has chlorine, chloramines, and heavy metals in it, which can poison and kill a fish. What water conditioner does is that it eliminates these things in the water, making it safe for your fish to live. Many people suggest using Prime water conditioner because it lasts so long and you don't need very many drops in your water.
Is your water heated? Betta fish are tropical fish, requiring tropical temperatures from 76ºF-80ºF. If he's sluggish and not eating, this could be the reason why. Just to let you know, you should not heat a tank smaller than 2 gallons. For a tank smaller than 5 gallons (but at least 2 gallons), it is recommended to get a heater that is 25 watts to avoid any dangerous temperature spikes. Any smaller and the temperature can spike and boil your fish alive. That leads to my next question:
How big is your tank? It is recommended to have a betta fish in at least 2.5 gallons of water...it allows you to have less water changes a week than a cruel, inhumane cube that's maybe less than half a gallon. Fish create a waste product called ammonia, which slowly builds in the tank. In a small tank, it builds quickly, and your fish can be poisoned with his own waste and get ammonia burns. In a 2.5 gallon tank, only need to change his water twice a week - 1-50% change and 1-100% change. Anything smaller, and you really need to change his water pretty much every day at 100%.
But for now, please answer the questions in the thread I have posted, as well as the ones I have just asked.