Green water sucks!

[AndyL]

Ok, need help... Obviously I'm missing something major since this has reared its ugly head again.

Frag tank has turned green again (6 weeks after the initial bout). Started going a bit cloudy last week, did 3 35g w/c's (tues/wed/fri), replaced my rowaphos/nitrasorb and carbon. Looked good again, this morning it was green.

Tanks a 55g, 20g sump, 2x175w halide (14k's), Coralife 220 skimmer (pulls about 175ml of goo / day), 20ish lbs live rock (not including the big hunks a few of the colonies are on). Fairly substantial amount of chaeto and grac.

With the rowaphos/nitrasorb and all the macros, never see measurable amounts of PO4/NO3, Only test thats showing a bit off - is pH is running a bit low - in the 7.9 range.

Anyone have ideas? Going to try changing bulbs, they're only a few months old - but they did get changed not long before this problem started popping up...

Andy