Could my Sg be impacting my Star Polyps?

Hi all,

Around 3 weeks ago I bought some green star polyps - looked great in the
LFS - got them home and they didn't thrive....some came out but they
never fulfilled their potential....tried every position in the tank and
every flow option but no go....So I spoke to the LFS today and they
agreed to take them back....they checked them out and believe that they
look healthy and viable....and exchanged them for another "thriving"
colony...now it looks like I'm going through the same thing again - some
polyps extended after an hour or two but then retracted and now no
action at all....

Water tests out as follows so it does not seem to be a quality issue:-

Temp 27C - a little warm I know
Sg is 1.026
pH is 8.4
Alk - high end of normal
Calcium 500
NO2 is 0
NO3 is 0
Ammonia is 0
PO4 is 0.25

Tank is a 15 gall/60 Litre with LR, a Protein Skimmer and 2 Power Heads
(can't remember the exact spec of both but one does 550L/h) - there is
plenty of water movement.

The lights are maybe not as good as the LFS lights but they didn't seem
to think this would be an issue with Star Polyps but suggested I swopped
the T8s for T5s at some point - the lighting is currently 3 Watts per
gallon with no room for further tubes.

The only thing that did come up in conversation with the guy at the LFS
is that their Sg is 1.023. I'm wondering if this might be the cause of
my problems.....as I'm currently only stocking with stuff I'm buying
from them I'm thinking that I should try reducing my Sg to the same as
theirs....what does everyone think?

Gill