When your fish meets another fish they swim together in a pair for a while. If a third joins them, one breaks off and joins another fish and swims with them.
Then they swim in threes.
And so on until there are two groups of fish forming two separate shoals, swimming around the room. The challenge is not to slow down as the groups get bigger, not to become a sort of weird huddle of people waving their hands together, but really to look like a shoal of fish, and perhaps more importantly to fee like like a shoal of fish. Part of a shoal of fish.