If you get it from a drug store I think it's 3%. You can add 2 to 4 tablespoons per gallon. You can get food grade that's like 30% but it's hard for me to find it. Remember it's just putting more oxy in the water, nothing in it that hurts plants. Rotting roots takes oxy out and you put it back. I used it in every rez change through the whole grow. You really only need it for a short time but keep using it.

Peace. Let us know.