Do you mean the first, or 3rd chamber, because if your talking about the 3rd chamber ( the one on the left side, if im standing there looking at it from the front) then thats where the stock pump goes, and the mj 900 is almost the same as the stock pump. If I were you, I would go with the mj1200!
I was talking about the third chamber. I realized after I posted that people were replacing the pump on the 14G Biocube with the mj900. I am considering a hydor flow but I think I'll see how it runs with the stock pump before I replace it.