i would think even with filtration, you would periodically want to change the water just like you would with a fish tank to remove amonia and other buildup like nitrates and what not. You only change out a portion of the water though not all of it. With water in the hydroton level you will get beneficial bacteria buildup that will work on some of the chemical build up but you still need to remove some of it through water change. Don't take my words on it, but it just seems logically to me.