DrBDC - I think it's MH vs. PC. You might not have grown it during the first 6 months b/c your tank was still "breaking in." It doesn't do as well under super bright light.
When I had the 120, the coralline growth in the sump was insane, but it was "normal" in the tank where the MH were. When I moved the MH to The Tub, the coralline exploded in the 120.