flush means to flush/clean out old food. always make sure you have pebbles 2" for when you have to flush, if your plants build up too much exces food, or when your in the last 2 weeks of 12/12. to help taste,get rid of nutes from the weed.
i dip mine in a dustbin full and drain.a few times. clean stood water. not from the tap. i collect rain water .thats what i use,or if you use tap water. let it staen for a day or two b4 using.for watering or flushing.. i only flush in the last 2week, unless i have to .