The type of tap water, the brushing, the balancing, has nothing to do with plaster discolorations and proper curing. Severe gray mottling discoloration (which is very smooth to the touch) is due to overly late hard troweling and the addition of excessive amounts of calcium chloride to the plaster mix, and will not be countered, offset, prevented, by any kind of various water chemistry balancing. The plaster compounds (material) are affected by improper plastering practices. The addition of calcuim chloride to the mix affects the curing and hydration of the cement. Calcium Chloride speeds up hydration of some of the cement components, and slows down other cement components, which leads to discoloration mottling. And late, hard, and dry troweling will darken the cement color by reducing the water content. That is what affects the "curing" rate of cement, not tap water or water chemistry balancing.