Yes it is. H2O2 breaks down around the 3 day mark...even sooner if there is a lot of algae, bacteria, ect. to combat. If everything is OK, and normal, it should be breaking down into the water around 3 days or so, due to slowly releasing its extra O2. Remember its natural so if you add the 3% dilute every other day (to battle the rot) it wont hurt the plant.
I suggest buying a large container of 35% food grade H2O2 (use caution), and dilute it down to 3%. To dilute the 35% H2O2 to 3% mix at a 1:11 ratio, 1 part (1 quart aka 250ml) to 11 parts (11 quarts) distilled water. If you get three 1-litre bottles that are black (to prevent light from entering), you can fit the example I used perfectly of 1qt. H2O2: 11 qts. distilled water..