ripple question...

[ALang]

I think that when your overflow isn't working for any reasons, at least you have a pwrhead helping you aerate your water, that is, if the power is still on, or if the said pwrhead is plugged into a battery backup system.
Or when there's a dead organism in your tank (unnoticed) and is using up your O2 at night to decomp, your overstocked fishes might die of oxygen depletion, etc.... Many reasons to just be safe, and point one at the surface of the water.

[lastlight]

Sometimes I find the scum likes to take refuge in little stagnant zones on the water's suraface...especially in corners so I find having at least one aimed slightly towards the surface helps with this.