The screen are fabricated from Nitex No.200-74 nylon screen cloth (rated porosity = 74 micro) imbedded in to an acrylic frame. A screen of 155 cm area sufficiently distributes an outflow of 125 ml/min/tank so that no concentration of animals occurs in its vicinity. At this flow rate, screen porosity restricts passage of the smallest nauplii but passes food organisms freely. Gravity drains merge to enter a foam tower described below. Compressed air supplied to the tower is first dried, passed over activated charcoal,and then is rehydrated by bubbling through a column of distilled water.