I think one of the main keys is to know what the hardness level is coming out of the tap. You may be fine adding the tap water, but you might already have more than enough calcium. Where I used to live, the water was so soft I had to add a fair amount of calcium to the water. Where I live now, you can add tap water, pH balancing chemicals and chlorine and you're done. I'd rather not go with soft water because, to me, it is easier just to go with the tap.