One bit of advise I can give you is Nitrates should never be zero in a cycled tank. How are you testing the water.
As far as your questions:
1. You can vacuum your gravel, I would do 25-30% at each water change(divide tank into quarters and vacuum one quarter each WC)
2. You can rinse out the filter in aquarium water but not on the same day as your water change(To avoid a mini cycle)
3. Not sure about the Bioballs.
The very first thing I would do is figure out why your getting a zero nitrate reading.
As far as your questions:
1. You can vacuum your gravel, I would do 25-30% at each water change(divide tank into quarters and vacuum one quarter each WC)
2. You can rinse out the filter in aquarium water but not on the same day as your water change(To avoid a mini cycle)
3. Not sure about the Bioballs.
The very first thing I would do is figure out why your getting a zero nitrate reading.