Answer:
I am a bow that can never be tied or broken but can disappear. What am I?
I am filled with garb the price is free. Just return what you don't need.
No matter how little or how much you use me, you change me every month.
I am a busy worker who is a water mammal.
I am the minerals vital for your good health.
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.