Interesting ... A few days ago I added some Caulerpa to the Chaeto in the fuge to see how it did. As of this afternoon it's doing OK, but not great. Also the Chaeto is holding on for now.Also if you mix caulerpa in one will grow then stop then the other will grow then stop. I have 2 types of caulerpa and chaeto. For some unknown reason to me one will always dominate for awhile then die back and the others will take over then die back. Right now it's in a chaeto dominant cycle but I see my prolifera is starting to send out more runners. Also I'm no botanist but I believe that algae prefer ammonia over nitrates, do you think that maybe your ammonia started to get processed elsewhere in the tank before it reaches the sump?