I got a science question see if you scientists can answer them?

You just asked what 2-2 equals. The force exerted by the fish when accounting the force of the water is equal to zero, but one cannot be sure that he will not accelerate, for there might be outside forces acting upon said fish; causing it to either accelerate or decelerate. In a perfectly controlled environment, however, Nemo would still accelerate; the direction being towards the Earth due to gravity.