I have had zoas come back from the ashes many a time-don't panic. Like others said-do a dip and move them to a different location and see how they do. If ur mag is truely 1800 that's way to high. I battled with bipropsis algae as well and kicking the magnesium up helps but I wouldn't go over 1550 if I could help it and even that seems a smidge high. Unfortunately I tried manually pulling clumps out, kicking up the magnesium, reducing the temp, etc. and the only thing that was truely effective and long lasting was pulling the most infected rocks out and spraying them with HP, sit the rock in the sun immediately after spraying for about 3-5 minutes then scrubb with a hard Bristol brush to remove the bulk of the algae-rinse twice in rodi water then put it back in the tank-all the algae died out within 48 hours-it works. And I didn't adversely effect any of my corals or fish. My tonga rocks looks very white and the reel reef rock and fugi live rocks did not lose any of the purple algae-but a couple of the aptasia survived-lol-doh!!!
