[Delphinus]

I myself do one in, one out. It's a small catch-22 because two fans that each produce "X" cfm of air movement will yield you 2x"X" if they are either both in, or both out, but if in series then you only get 1x"X" cfm despite two fans. But nevertheless, the air movement is more controlled (i.e., comes in one side, exits the other). The difference I found with this arrangement seems enough to convince me. If I put my face in the outward facing fan, it's blowing considerably warm air.

But like Brad points out, it's only keeping the bulbs cool, it doesn't do a whole lot to keep the tank itself cool. For that, you really do need a fan blowing downwards onto the water. I have this over my sump though and despite it being an 8" fan it doesn't seem to keep up. All it does is make me have to replace 5g per day of evap loss! I'm wondering if a fan pointing downwards onto the water surface in the hood itself may not be a bad idea. .. I'm not sure yet.

PS. I should mention that in 3 or 4 years of fans in my hoods, I've only ever chucked out one fan, and that was after about 2 or 3 years of continuous use. And it was one of those bargain bucket fans from Princess Auto -- these fans aren't new. So who knows how many hours of runtime it had over it's lifetime. If you buy a brank spanking new fan I would expect you not to have to worry about garbage canning it for quite some time.