It the 24 long, 12 high and wide. How much was your fixture? I like the blue lights, cause this is next to my bed, and viewing it when the lights off would be nice! I'm gonna look into it!
Do the fish/plants not register blue? Or is it red?
The t8 does well in my other tank, seems to be growing my swords and stems and everything else quite well. Just the 15 is much much shorter.
What would be the reasoning behind choosing t8 or t5? I'm not sure i want to go led cause of cost.
I think it depends on the availability of replacement bulbs. Though T5NO's are a bit weaker on the PAR side when compared to T8's.
Check out the PAR data charts in this thread and see what works for you at 12 inches high.
Lighting an Aquarium with PAR instead of Watts