Thank you, Solo77.
I have a confuse question:
To maintain a FOT, example 4'X 2'X 2', 10 medium fishs,
it means that I should change the water to reduce the NO3 every week,
or i do not need to change the water, just fill up the volumn of evaporation water.
Is it possible to reduce the frequnce of changing water if i add live stone in FOT?