Phosphates AND nitrates come from everything we put in our tanks, starting with make up water. Tap water can contain both, the food we feed our fish & corals will contain phosphates, and the dying and decay of food and organisms is another source. Try feeding less, using RO/DI water, and use Kalk for a calcium supplement as it precipitates phosphates. Heavy skimming will reduce dissolved organic compounds, (DOC), and therefore reduce both as well... hope this helps. 
Oh ya... be patient... remove as much as you can harvest by hand, siphoning etc... if your lights are old and shifted in spectrum that can cause a bloom as well... also algae doesn't like to grow in areas of high flow... there's really no "one" answer... it's a combination of conditions...
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135G Mixed Reef. Bullet 2, 25 gal refugium, 2 X250W MH + 4X 96W PC\'s, DIY Calcium Reactor, Coralife 1/6 HP Chiller, Phosban, Tunze, 2 closed loops & SQWD\'s, Seios, Coralife 4 stage RO/DI & a bunch of other expensive gadgets... I may never retire, but I'm gonnahavahelluvanaquarium!
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