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The red tail could be correlated to the blood that runs from the head along the lateral line that ends at the tail.
When the fish swims hard (due to the fact that fresh water is not as boyant) there is a sort of whip lash in the tail. Therefore the force will push the fluid (blood ) to the rear of the tail making it red.
The above has not be checked for accuracy just a possibility.
This being said, looks more likely to be that skin disease as quick decision indicated above.
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