Answer: Using a water filtration system remove harmful contaminants from the water. Also having a water filtration system will make the water's ppm level 0. All feeding schedules start at a 0 ppm level. You might not have to use a water filtration system but it is the best water for your plants because you have no way on knowing what make up the start ppm in your water. Example: If your water out of the tap is 400 ppm you have no way of knowing what 400 ppm is composed of. If you have city water & you use beneficial bacteria or fungi you HAVE TO USE a water filtration/purification system or the chlorine/chloramines will destroy beneficial bacteria or fungi.