How many gallons is your tank?
Does it have a filter?
Does it have a heater?
What temperature is your tank?
Does your tank have an air stone or other type of aeration?
Does your Betta have tank mates? What kind?
Before your Betta became ill how often did you perform a water change?
What percentage of water did you change?
Do you vacuum the substrate or just dip out water?
What additives do you use?
What are your water parameters? Please give exact numbers. If tested by pet store please get exact numbers. "Fine" or "Safe" won't help us help you.