HOW GOLDFISH SWIMS
A goldfish swims when it exerts force against the surrounding water. This process is often accomplished by the goldfish contracting muscles on either side of its body in order to create waves of flexion that travel the length of the body from tail to nose. The vector forces exerted on the water by the motion cancel out laterally, but create a net force backwards which in turn propels the goldfish forward through the water. A goldfish generates thrust by using lateral movments of its cuadal fin and body. The scales of a goldfish also serve to streamline it, helping to glide the body of the goldfish more easily through the water.
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