The problem could be your nitrates.
Get a Red Sea Pro nitrate test kit. Reef Supplies sell it. I found the salifert test kit sucks when measuring below 5ppm.
Find out exactly what your nitrates are. I was having a problem with phosphates off the chart on a similar low fish bioload system. Once I got my nitrates to 1.5ppm I was able to take the GFO off line and now have acceptable level of phosphate without any GFO
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