I am using an ATB 840 V2 with my +/- 70 gallon system, and it does great. I was a little concerned when I first bought it that it would be too oversized, and wouldn't skim, but I have had no trouble at all. Really a great skimmer.
Hi, I have the ATB Small, it basically identical to the 840 model, when I talked with ATB, they changed the Eheim out for a cheaper pump and was able to lower the price to be more competitive. Go for it, skims like crazy on my 120 SPS tank.