A fish swims by moving it's tail (caudal fin) side to side. While each motion provides thrust and lift, it also creates drag when the fin is angled to the side of the body as a stroke is completed. It takes more than a fin to be fast. More tail area gives more thrust, so fast fish have longer lobes on their fins.

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Their fins and skin

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A fish swims by moving its tail (caudal fin) side to side. While each motion provides thrust and lift, it also creates drag when the fin is angled to the side of the body as a stroke is completed. It takes more than a fin to be fast. ... More tail area gives more thrust, so fast fish have longer lobes on their fins

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Fish swim by flexing their bodies and tail back and forth. Fish stretch or expand their muscles on one side of their body, while relaxing the muscles on the other side. This motion moves them forward through the water. Fish use their back fin, called the caudal fin, to help push them through the water.

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