To separate the nematodes from the mixture of foam and growth medium, the foam substrate was rinsed five times in a container filled with 2 L of water from the fish system. To achieve a homogenous distribution of nematodes, the water was mixed as a 1 mL sample was and diluted with water to 10 mL, from which the nematodes were counted in three 50 μL samples using a stereomicroscope. The total number of nematodes in the water sample was estimated from the average of three counts. Once the nematodes had settled on the bottom of the container, most of the water above them was siphoned off and the remaining nematodes were poured over a commercially available milk filter (diameter 229 mm, Vet Net Milk®). The nematodes migrated through the filter and accumulated in Petri dishes out of which they were taken to be fed to the fish.