Ah yes, the Hagen company strikes again with proprietary design.
I own the Hagen GLO T5 units on both of my larger tanks. Trust me, you need to buy the Hagen T5 bulbs only. You will not be able to easily use any other maufacturer's bulb in the unit.
Fortunately, the bulbs made by Hagen (LifeGlo, PowerGlo, etc.) are great lamps. Check out the higher K rating bulbs (PowerGlo) for a more actinic quality blue-effect and deep water feel. The LifeGlo units are great for planted setups. I use a mix of both to achieve the broadest spectrum possible.