A method for maintaining separation of old and new dialysate in a dialysate preparation tank is disclosed. The new dialysate is stored in the tank at a first temperature and withdrawn from the bottom of the tank. The dialysate is circulated through a dialyzer, and then returned to the tank at the top of the tank at a higher temperature in a substantially non-turbulent manner. The temperature differential between old and new dialysate and the manner of withdrawing and reintroducing the dialysate to the tank cooperate to form discrete zones of old and new dialysate without substantial mixing of the dialysate in the two zones.