No, the blackout won't kill the plants. I can't remember how many days I did mine, maybe someone in here can shed some light on length of time. Some of the plants did lose some color, but sprung back quickly after the lights came back on. As for the H2O2, no, it's not 3% of the tank. You use 3% H2O2, which you can get at Safeway, etc. Look on the bottle, it will tell you if it's 3%. If you read the article that I had a link to on my other thread that I just posted, the guy on there uses 4oz in his 55G tank.