If you are dosing H2O2, you have to watch the total quantity not to exceed the threshold that will harm fish. But if you just spot spray the plants, it doesn't matter because H2O2 breaks down into O2 and dissipates quickly in the atmosphere. I don't know the dosing level as I only spray. Spraying will kill BB in contact, but you are not spraying the filter or the substrate, so it won't upset biological.
