At home I have softened well water, and my fish did not do very well in it. As I understand it there are some chemicals in water softener that may not be healthy for fish. He was lethargic and had no appetite, even when I put conditioner in it. I switched to spring water from the store (2.5 gal jug for $3). He instantly perked up. I'm moving back to school though where I have city water and can switch him back over so I don't have to buy spring water any more. Your water may be just fine for your fish, so try that first. Mine didn't seem to like it :/
I have a 2.5 gal and I do a 50% and a 100% a week.
You should really condition your water, no matter if it's from a bottle or tap. It won't hurt and if there is any chloramine, chlroine, ammonia, or heavy metals in your water it makes all the difference.