I messed around with H2o2 for bugs, I found a post of mine from years back and I was using 50ml per litre on the leafs, I'm thinking OMFG 50ml but I must have used it or I'd never have mentioned it on the forum, I can remember my hands getting slightly burnt with it (your skin goes a whitish colour) I also remember it fizzling when I put it down through the perlite.
I lost all my growing notes to the authorities, tbh I was a bit surprised to read 50ml, I've only a 250ml bottle I use for seeds or I'd give it a try?
Or... 5ml per 100ml is equal to 50ml per litre.
I'm going to mix up 5ml to 100ml and test it again it sounds ridiculously high?
Yeah I used to bleach my hair using 9% peroxide when I young, I know how bad it burns
50ml of 17.5% per liter makes a 0.875% solution, which is not ridiculously high considering this other post I mentioned above claiming to soak the seeds in 3% solution, but then again it might be a different story when used directly on plants. So yeah the 70ml you used might have burnt your plant I guess...
I found this site that gives directions for using h2o2 on all stages in case of damping off, you might want to check it out:
http://4urbanfarming.blogspot.com/2012/01/pythium-root-rot-and-damping-off.html
But I wouldn't really consider using it unless I run into some algae etc. problems... Maybe only while germinating my seeds, which I'm currently looking into
For your 17.5% h2o2 just mix it with 5.5ish parts water and you'll roughly get a 3% solution. And from there on, I would dilute it even more according to recommendations. For example, the site I linked above says 1 teaspoon (5ml) of 3% h2o2 for 16.9oz (500ml) water for germination. Which makes it a 0.03% solution... just like you do for germination. But they also recommend using it while watering Rockwool... so that is probably a safe spot for young plants. In theory, soaking only the seeds in 3% directly MIGHT BE safe for a short period of time. Which again, I wouldn't dare to take the risk