With the magnesium method you want to bring your tanks magnesium levels up to the 1600-1700 range(raising it no more then 100ppm a day), and keep it there till your hair algae goes away. You want to use a liquid magnesium because the dry stuff is full of sulfates, and you have to use a fair amount to get your mag up high enough. My only concern would be your mangroves. I am not sure if they respond as negatively to the mag as the hair algae does. When I did the mag treatment in my tank, my cheato made it through just fine, so it doesn't harm all algae/SW plants necessarily, but I couldn't say one way or the other for the mangroves.