Since the majority of our beneficial bacteria reside in our filtration system, I have no problems removing 60% of the water at a time to get our nitrates low. We can't remove any more than that at a time because the pacu have to swim sideways with the water low, and we run out of hot water at about 200 gallons. We change out another 60% the next day, and that keeps our nitrates < 20 ppm for 6 days