The underwater villages in this Roleplay could have barriers built to protect them from predatory sea animals. This causes a small ecosystem to form, and to stop the fish from overpopulating, the sea people would gather some of them up and either eat them or put them on the other side of the barrier.
As for the plankton, the art style of the show makes the water clear, but there are plankton and sea life and what not.
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Because it's the weekdays the IC would be rather slow