probably 180g? The newer gen 2 have a better pump/volute etc and can handle a bit more volume. I've got a 160 on my 130 (90g tank, 40 growout/sump), and would get a 180 for a 120g-150, and a 200 for up to a 180-200g. The build is fabulous, and the energy use is minimal, very quiet.