Sump pump size...

[Lampshade]

sphelps is right with the problem waiting to happen. I'm currently runnign that setup, since i only have 2 holes drilled in my tank and i'm running a herbie. A herbie emergency drain should be bigger than the original, mine is not. I've got the main fairly restricted, but if it was to 100% plug, there would be problems, i don't think the second overflow could handle 100% flow. It would top the overflow and still make the drain, but my tank would be near/at 100% full.

As for the stagnant second overflow, i have a 1/4" hold drilled about halfway down my emergency drain, keeps a bit of flow, but still lots of sediment at the bottom of the overflow.

But yes, i would recommend another hole drilled to run a herbie (a 2" if your main return is 1.5"), it's silent. There's lots of other setups around too, i like the stockman, or the Beananimal mentioned is a great one as well. All preference.