i have something like dust (brown) on rocks and sand
it's not attached to them
cause when i make water current to them they disapper and flow on water
what is it?
and how to get ride of it?
If you are getting some type of sediment (which I would assume is what is happening) on your rocks and sand I would try increasing the amount of water flow... ie..adding powerhead(s). This should keep things stirring until it can be filtered out.
I agree with two11devan, tap water is often the cause of these kinds of algaes. Have you tested for phosphates and/or silicates?
Silicates are primarily the cause of brown algaes, diatoms and dinoflagelates.
Phosphates are typically the cause of green algaes. People using bottled spring water will have many problems in that regard.