Yeah that's when I add the h2o2, about 4/5 days from initial watering. The smell is a great indicator when to do it.
From time to time I will add in some lemon juice for carbs, vit c and it seems to perk up the benes. Plants like it too, I think because the myco uses the lemon juice carbs giving the plant a bit of a break.
If your city water supply uses chloramine (chlorine + ammonia) you can use vitamin c to neutralize. Most vitamins are good for plants and I have been known to crush up some old multivitamin tablets for amending soil.