In nature, currents are caused by wind. In some areas on the reef the current is ever flowing 24/7. Fish adapt to the level of current. I would leave the ph's on 24/7. By leaving them on it will help clear debries and toxins that some corals tend to release at night. IMO water flow is the third most important compenent of a healthy tank. Following, water quality and lighting(if it's a reef tank)