I was asked to come here for a look. As has been pointed out it is more likely a lighting issue and maybe iron, when dealing with cheato. Cheato is mostly only a chlorophyl
a user. Chlor
a has its spike of at about 440 nm and a medium peak in the 670 nm range. Chlor
b is a accessory pigment, unlike a which is not an accessary pigment. b can only trasnfer energy to a, as b can not creat a light enery to chemcial energy reaction, where a does. This thus confines its range making it more picky on light, where they seem to do better under reder light.
An interesting read on Chaeto
Production within dense mats of the filamentous
macroalga Chaetomorpha linum in relation
to light and nutrient availability
http://www.int-res.com/articles/meps/134/m134p207.pdf
Yes, you may want to think about dosing iron, as a lack of iorn will cause chlorosis.
Some more reading;
Photosynthesis and the Reef Aquarium Part I: Carbon Sources
http://reefkeeping.com/issues/2006-10/rhf/index.php
First Iron Article: Macroalgae and Dosing Recommendations
http://www.advancedaquarist.com/issues/aug2002/chem.htm
Second Iron Article: Iron: A Look at Organisms Other than Macroalgae
http://www.advancedaquarist.com/issues/oct2002/chem.htm
Phosphorus: Algae's Best Friend
http://www.advancedaquarist.com/issues/sept2002/chem.htm
What many have been using to grow Cheato is a bulb from Homedepot, which is a 2700k spiral compact florescent spotlight bulb with a std crew base. This bulb is round with a built in reflector and has the appearance of a "PAR" halogen bulb. For what ever reason, this bulb seems to work well, even though it is much higher in the red. I do not know how many houses I have been to in the last year where Cheato seems to grow like weeds under this light.