First off, light is NOT required to break down hydrogen peroxide. Pretty much any contaminant will begin the decomposition process, and it will do so very rapidly under the conditions in our fish tanks. Secondly, bacteria are quite resistant to peroxide. Essentially all aerobic bacteria contain an enzyme called catalase that decomposes peroxide in order to utilize the oxygen, and is able to do this at relatively high concentrations. I've seen studies showing that bacterial growth stops around 300 mg/L, but I don't remember what concentration is required at what length of time to completely kill it. So assuming you dumped a liter of peroxide at 3%, you should be fine with a tank of at least 30 gallons (as far as bacteria goes). There may be a lag time for the bacteria to recover, but I doubt the tank has been sterilized.