What? There are many with HC in 75/125g tanks which has very similar depths. As long as you don't cheap out with the light like many do you could do what you want with a 3-bulb T5HO. 6 will be a little much, IMO. You could get the fixture as you state in case you go SW and just unplug a couple of the bulbs or raise up off the tank. I never had a problem just unplugging bulbs but some may be grouped and need all bulbs to be connected. This is probably different from manufacturer to manufacturer.