I also read that if you use h2o2 in res, you will have to always you it from that point on for every nute change because it kills some beneficial stuff the roots or plant does to fight off disease. The roots/plant becomes dependent on it, if I understood that correctly. I don't think using it during a flush is as drastic, but I'm just sayin what i read. Wish i knew where... Whatever it was, it scared me away from using h202 until I am ready to always keep it on hand.
That is just what i read someplace, so i don't use h202.
I also read that "food grade" H2o2 is like 27% or something concentrated. I'm having a hard time finding it at stores, but i'd like to have a gallon of the food grade stuff on hand just in case...