Weird Black Lines

[snake007]

Yesterday my betta had black horizontal lines going from head to fin, today their gone, she had these black lines since I got her but they've just vanished.
Why is this?

 

[Louiseness]

They're stress lines. Normal when they're feeling stressed (obviously) from being introduced to a tank or when they're aggressive.

 

[sausagewinkle]

They are known as stress stipes, females and often some males will show these stripes, they are more dominant depending on their colour.
Its nothing to worry about, you will notice they will come and go