Answer:
Gets rid of bad ones, short and tall. Tightens when used, one size fits all.
What goes in the water black and comes out red?
I'm not man's best friend, I'm their enemy, I can mark your end, yet you do not see me, I am very small, but very tough, If you have me, then you have it rough.
What flies forever, Rests never?
My teeth are sharp, my back is straight, to cut things up it is my fate.
All about, but cannot be seen, Can be captured, cannot be held, No throat, but can be heard. Who am I?