I have mh for the same reason, that awesome shimmer. As everyone has said 250w will be way too much. I've used two 175w on a 120g hanging about 12" over the tank. I currently have three 150w on a 220 which is 30" tall and it's great. I have two 70w on a 180 and found it's plenty bright for the low light plants I have and for me to enjoy the fish. With the 70w's I mounted them at the ends of the canopy and let them shine across the tank. That allowed me to use 2 instead of 3 to cover the 6'. The general rule is 1 mh every 2'. There is some slight shadowing in the upper center but I like to play with lighting/shadow effects so it doesn't bother me. I've found that mounting the lights at the ends and letting them shine across the tank enhances the shimmer. Also, the shimmer is caused at the surface so you'll need powerheads or filter outflow under each light to agitate the surface or you'll have a bright tank and no shimmer. For no plants go with the 10k bulbs they are clean crisp white light. Some fish look good under 14k (slight blue tint) and 20k (crisp clear blue) but the bulbs are expensive so it's a crap shoot if you can't see them under those lights first. I'd stay away from 100w bulbs as they can be hard to find unless you are looking for a yellow light (3.2k). The most common aquaria bulb watts are 70, 150, 175, 250, 400. Also if since you're FW you don't need to change the bulbs every year. They will loose intensity and the color will move to the yellow side as they age but it will be a slow change. I generally don't change them until they burn out or I want to change the color temp (#K). Saltwater with corals is another story.
MH FTW!!!!
