Depending on the bulbs you choose (and there is a giant variation between bulbs), you can use either 2 x 175w or 2 x 250w. 400w is overkill for that size tank.
now what is the big difference between single ended and double ended? I see the double ended HQI bulbs have slimmer reflectors. The With is thinner and better for under the hood but will i have any advantages with either of the two?
The only difference is a DE bulb must be used with a reflector that has a UV
shield where a SE bulb is already incased in a UV
shield. There is no difference in heat output.