Actually, the that is the size of the bulb in 1/8th of an inch. A t8 is 1 inch in diameter, a t5 is 5/8th of an inch in diameter. And actually, the smaller the number, means that they are usually more energy efficient. t5 normal output, tends to put out more light per watt than t8 normal output. That is why there has been a movement towards the t5 high output over the t8 high output. They put out more light per watt and you can fit more in the same area. I have never seen a t6 bulb available for an aquarium either.
Brian