Why front / back versus side to side, side to side would be more spread length wise i guess it depends on the width...
for a 24x24 cube would you suggest side to side?
If you have only 1 light it wouldnt make a difference. I have a 24x24x24 tank, I have just 1 radion and its running side to side but if I added a second light I would run them front to back. The light is 11.8" in length so running them side to side I would think would focus the light in the middle but if you placed them front to back it will balance out the light & you can center them better since they are 7" in width.