k, but wont it shock the plant to change its water?
No, it shouldn't. I change it out every 2 weeks. Just empty it, mix the nutes, Ph the water and fill it back up. Matter of fact I leave the res empty for about 10 minutes before I refill it. They like the air to their roots.
Every 2 weeks I flush it, add 1 tsp CLEAREX, cycle it for 30 minutes then refill with the nutes of the week. (If you have a ppm meter (you should), put in the Clearex and check your ppm's. When it STOPS going up, drain your reservoir.