Actually, I thought 15g tanks were the same footprint as a 10g, but taller, but I was mistaken...lol. I've never had one or seen one, but for some reason I thought that. Those must be 15g high tanks and the 24" is the standard like EMC said.
Yep, thats a Compact Fluorescent bulb/fixture and not T6. T6 fixtures are regular fluorescent lights (tube looking bulbs) and T6 is just a smaller diameter bulb than your regular T12 shop light bulbs.
The Kelvin rating is good too (8000K)...perfect for plants.