Similar experience... Despite having a LR Hanna colorimetre to test for Phosphate, I cannot determine with any consistency when to change out the GFO....so I change it every month.
In a continuing battle with cyano, I'm going to run larger quantity of GFO and see if that makes any measurable or visible difference. I'll also experiment with increasing the change frequency. From what I've read online, GFO has the capacity to absorb a lot of phosphate very quickly...but that might apply only to those with high phosphate levels to begin with.
My phosphate levels are always quite low, so it is likely the GFO lasts longer.
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank: http://www.canreef.com/vbulletin/showthread.php?t=28436
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