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Having air stones driven by back-up, battery powered air pumps is a good idea in the event of a power outage. Such air pumps can save your fish in the event that you lose power and your power filters or air-driven filters stop working, especially if you’re at work or away for any length of time.
If you’re in the market for a new air pump, consider the Silent Giant by Hydra Aquatics. It might just be the quietest aquarium pump on the market. You can see a video of it being tested against a couple of other comparable pumps, including Danner, on the Aquamanknox YouTube channel here.