Acan mature heads not opening

[Jaweeha]

In my 90g tank, I have a few different types of acans, and up until about 3 weeks ago, all were doing great, but now the mature heads aren't opening, even though the baby/new heads are. There is on loss of color, and other than being closed look A-OK. Lighting is 2x 250w halides, the acans are about 2 1/2 feet below them.
SG: 1.025
pH: 8.2
Alk: 8
Cal: 420
NO3: 0
PO4: 0
Mag: 1300
temp: 78-79

 

[greech]

I have had this happen too and while I have nothing but my own observations to base this on, it almost seems like they do this to give the baby heads room and light.