The problem is PAR per square foot. NO output T12 bulbs are actually very efficient. The problem is that you can not pack very many of them over top of a tank. MOST of the light emmited from the top and sides of the bulbs is wasted. (actually over 3/4 of the light)
So that brings us to T8 bulbs. You can certainly get MORE of them over a tank and therefore increase the PAR, but most of the light from the top of the bulbs is wasted, as the bulbs themsevles cast their own shadow as well as catching and scattering a good portion of the reflected light. We still waste a majority of the light.
That brings us to T5 bulbs. They are VERY narrow and we can get a good number above a tank. Better yet, we can build very slender parabolic reflectors that take a large portion of the light from OVER and BESIDE the bulb and get it down into the the tank and still fit a decent number of the fixtures (bulb and reflector) above the tank.
If we tried to use parabloc reflectors on T8 or T12 bulbs, they would need to be HUGE to work properly and you would not be able to fit enough over the tank.