The tank is fitted with a drain valve (D), which also serves as the input port for a "saltplug" of saturated brine solution. When the in-flow is switched off, this "salt plug" of 2 or 3 l volume is gravity-fed into the drain and the valve closed. Living larvae will swim to the water surface and thereby avoid the dense salt solution, which traps the dead and moribund. After a few minutes, the drain is opened a fraction to eliminate the "salt plug" and the dead larvae, much in the same way as dead marine fish eggs are accumulated and eliminated from incubators.
Larvae are supplied with the required food ration by peristaltic pump (P) from a cooled and aerated algal reservoir (AR). The quantity and rate at which the ration is supplied depends on the number of larvae, the size they are and the flow rate through the tank (refer to parts 5.1.4.1 and 5.2.3.2). Mean shell length can be determined by taking daily samples, but survival is more problematic. An estimate of mortality can be obtained by sub-sampling the volume of saturated brine collected after a "salt plug" and counting the dead larvae it contains. This can be done at 2 to 3 days intervals to keep track of survival.
Internal surfaces of flow-through tanks need to be cleaned with a soft-bristle brush, attached to a pole of suitable length, at least once during the culture period of a batch of larvae. The flow rate should be increased during cleaning to flush out the dislodged debris.