What function are you trying to use? Doing a quick search for a bandpass filter and the functions have the cut-off frequencies as inputs. So I am not sure what you are trying to get at.
How can I decide the cut-off frequency (low and high) in band pass filter?
That's a pretty ambiguous question. That's like saying, "How fast should I drive?" The answer to both questions is, "It depends."
I accidently sent it sorry. My problem is with the cut-off frequencys. I am trying to set it lower than the 500 because my fs is 1000 and with that lower frequency should be lower than 500 and maybe it can be 450 and higher can be 485 right?
Note: I am trying to filtering noise from the sinusoidal source.
Thank you for all of your answers.