Nitrate and phos readings with that large of an algae outbreak will be misleading. The algae would have no nutrients to grow if the levels were truly that low, the algae is consuming the nutrients in water column which is why the tests come back so low. If your nutrient readings were actually that low I would expect to see dinos showing up and the corals doing poorly.
A refugium growing cheto may give you good long-term results as it outcompetes the nuisance algae in the display.
