Algae as you know needs a food source. Both nitrates and phosphates contribute to this food source. Many times your readings of these 2 might not seem high do to the fact that the algae is consuming it faster than it can enter the water colum.
The first thing I would check is your source water. Most stores that sell RO water do not do the required maintenance needed to keep the Nitrates and Phosphates low.
Secondly what are the current nitrate and phosphate levels in your tank?
Third what Macro are you using in your refugium? how is it lit?
J
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