Actually some local club members of mine have successfully used high magnesium levels to kill off hair algae. At somewhere between the 1500-1600+ppm range the hair algae turns white and dies. It still needs to be manually removed so you don't have white strands everywhere but once you get it that high, it seems to work. None of the folks I know who have tried it have had any troubles with their fish or corals and many have some pretty sensitive hard corals. I've never tried it personally but it does seem to work.