35% at 1.7ml/l every 3-4 days is what I use, keeps the res. clean (no fungus etc.), it oxygenates far beyond what air stones can do and also keeps your rootzone healthy. I do not have to clean my res anymore either. My plants love it. Downside is that it kills all live things in your soup i.e. beneficial microbes as well. It's a tradeoff, but ultimately gives you a more consistent environment to work from. Give it a shoot, my plants love it, just don't overdo it
Good luck,
Unity