As pointed out, the leaves indicate over-watering or, more precisely, root suffocation.
But I think it's linked to your watering with a H₂O₂ solution, which is a very bad idea because you destroy all the micro life in the soil and if the dosage is very high, can burn the roots.
For the preventive treatment of root rot, on the contrary I would have reinforced the soil with enzymes.
For the time being, I think a flush is needed to get rid of the hydrogen peroxide and restore microbial life as quickly as possible. You'll need Elycitor or equivalent.