About you wanting to flush every 3-4 weeks, I wouldn't recomend it. In hydro you want to slowly build up your ppm's. I would only flush after 1-2 weeks of veg depending on your strain. When doing a final flush those expensive flushing agents are a waist of money if your using 0ppm ro water. If your using tap it works better with that agent. Flushing agents are only good to use if you under estimated your bloom time and need to flush quick or you have highr ppm water.