Husbandry on days between water changes consists of restoring the food cell concentration to compensate for cells eaten in the previous 24-h period by adding sufficient freshly harvested algae. A sample of water is taken from each vessel and the remaining algae cells (residual algae) per unit volume are counted, either by using a microscope with a haemocytometer slide at x100 magnification, or, more easily, by a coulter counter or similar particle counter. Where it is impractical to determine residual algae, either a full or partial ration of food can be added on intermediate days between water changes based on the previous day’s ration fed.