Do you have any direct sunlight can reach the tank at all? That could promo hair algea growth.
I used to have the similar problem until some guy told me to move all coral to a QT tank and keep the main tank running without any light for 3 months, I end up solved the issue that way. However, you will have a big enough QT tank and also have te time to keep up the water quality in the QT tank.
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