Try to do a few other things before you syphen out your water. Use a pump to blow off the rocks to help get the detritus into suspension in the water column. You dont mention if your using bio balls, rock rubble etc, but a cleaning of those in used water helps to lower nitrates. I may be wrong to, but i fought the brown algae for a while until my water quality cleared up. Meaning, less nutrients floating around. My LPS didn't like that much, but it took my nitrates down to below 10 , and mine were always running in the 60 range.