I just bought one of these to replace an old maxi-jet 900. Very impressed so far. I got the 1200 gph model and its quiet as a mouse and is putting out some good flow in my 120 display. Its very nice having a super strong magnet built right into the base of the power head too. The thing I'm wondering, if anyone knows the physical dimensions of the different models? I know there are different models with different flow rates but are they the same size with different size impellers or do the entire power heads get bigger as they increase in flow rating?