the way i change my nute every 2 weeks is , i have another res. that i get a gallon jug and add water til its 5 inches from the top of res, and mark off where it would be in the other. and the way i fill and drain is the same with a kerosene battery operated pump. this time of the year every hardware store should have one. so i fill up in my bathtub and measure my nutes right there. use your ppm meter to make your nutes exact , very easy once you do it the first time. do you have a ppm chart? and when your not doing anything check these grows out. www.urbangrower.com