I'm assuming that you wish to flush for over-fertilization, if however you are flushing for maintenance, the following advice is still applicable. First, make sure that the water is neither hot or cold. *Distilled water gallons r good for a quick supply of neutral water if you don't have any that's been sittin out gettin aerated by a air pump) Slowly (if soil) flush 2 times the medium's hold. example: if the bucket is 5 gallons, pour 10 gallons of water through it