I currently have the same tank, Oceanic 30 cube, setup at my daughters school as a mixed reef and I am running 4 24" T5's over it and the tank is rocking. Wait I did to make up for the length was to extend the canopy about 4" over the back and ran the bulbs front to back. I will be glad to take pictures next time I am there if you want to check it out. The thing I like about the T5's are very little heat which equates to no fan and slow evaporation.

-Greg