Seriatopora with no polyp extension

[Kostis]

Hey everyone. I have a seriatopora coral in my tank which even though it looks healthy and grows quite fast, its polyps never expand the way I see in other tanks.

The tank is 150 liters with 6x24watt T5 bulbs. The coral is in the upper part of the tank. The flow in that part of the tank is medium.

water parameters:

alk: 1025
kh: 8.6
Ca: 400
Mg:1320

wc: 35% every 20days

Any ideas are more than welcome. Thanks in advance.

 

[toothman]

possibly too nutrient poor, not enough food fed.

 

[Kostis]

Do you think that increasing the quantity fed to the fish daily would do the job, or should I supplement with something more coral oriented?