You can reintroduce organics easily with Recharge, Fish Sh!T, et al , so go ahead and use the H2O2.
I suggest getting a quart of Food Grade
@30% H2O2 and mix that down to 1-2% (i.e. six times the amount of water to H2O2) using that should rid you of the issue for the time being perhaps permanently. The problem with the pharm H2O2 (3%) is the stabilizing solution is SLIGHTLY toxic to plants at those levels (i mean slight). You can use it in a pinch but not the best.
You are on the right track.