Are you soil or hydro. Both require differant flushing methods. Basicly you want to clean out all the old nutrients. You can use a clearing agent to help out. With soil I flush twice once before flowering and once before harvest. The timing is important to get it clean pior to harvest or its too late. You can use a ppm nutrient meter to chech how well you are flushing. General rule is flush 3 times the amount of proper ph water to flush. ie. 5 gal bucket would need 15 gallons of flush. Hope this helps I m new at this also. Good question