Big - Good point about filter failure, I hadn't thought about that. I'd add that if one filter failed in a series hook up you could have a large mess. For instance, the second in the series fails but the first one keeps running. Depending on the filter you'd have the first one continuing to pump into a non-operating second filter. Unless it was a closed system I'd think the water in the second non-operating filter would overflow and the first one would keep pumping until the tank got below the intake....then of course it would run dry and you've potentially ruined that filter too. Personally I run two filters on both of my tanks, partially because I lean toward over filtration, but more that I travel and either tank could "limp" along with either filter running by itself.