I think that free will ends up in that randomness of neurons firing. That can't be really predicted and steered. Free will is essentially freedom from manipulation/overriding external to the entity having free will. Theoretically that doesn't even need randomness.

And I don't think those fish free willed themselves. They just grew more random due to randomness inherent in our universe, despite us trying to force them into being exact. This was example of randomness of complicated biological processes, not of free will.