Your plant is hungry. 300 PPM is much too low. Take it up to 1000 PPM. The yellow on the lower leaves tells you that it is nitrogen deficient. A plant will move nitrogen from lower leaves to higher ones when it is hungry.

I'd avoid using the peroxide. If you must use it, it should be the food grade kind sold at the hydro shop. Some info on this can be found here
Using Hydrogen Peroxide in your Hydroponics Growing System