I would suspect that you were giving them to much light. I keep all my zoas in the lower 1/4 of my tank and they have always done great. The problem with advice from a forum like this is that you will get a lot of conflicting responses. I have never had a problem growing zoas and running GFO.
Moving the from the top of the tank to the bottom 1/4 of the tank. I worried about stopping the use of GFO. I do my water changes every two weeks of 20% and that hasn't lowered the Phosphates. Only when I use it did the levels drop rapidly.
About 4 months