Have you ever gone out of town for a couple of days, returned, and discovered that the filter you have on your tank(s) failed at some point (e.g., just stopped working)? Many aquarists rely on a single filter to both clean the water and provide surface agitation. If you’re in that group and the filter fails, your fish will suffocate long before they’ll die of poisoned water. So how do you address that? Filter redundancy.