Now compare the wattage of the NO (Normal Output) or Standard tubes to that of VHO (Very High Output) tubes. The wattage ratings on the NO/Standard tubes are much lower than that of the VHO tubes of the same length.
As you can see, instead of having to use 6 - 40 watt NO tubes, you can achieve the same or similar results by using just 2 VHO tubes. See why VHO's are the industry standard in reef lighting? The actual number of tubes necessary to achieve that "3 to 5 Rule" makes VHO's a natural!
Fluorescent lamp labeling, such as that of the basic stock NO tube widths of "T-8" (skinny) and "T-12" (fat) can be confusing. All this means to us is that the physical size of the fluorescent tube must match the hardware for which it was designed. Rarely can you replace a T-12 in a T-8 fixture, it just won't fit! That's all these "T" designations refer to, the diameter of the tube. Generally, smaller systems and their corresponding hoods use the T-8 diameter bulb, while the larger units have the elbow room for the larger T-12's.
Using off-the-shelf lighting would dictate 4 to 6 tubes spread out over the open top of the tank. Open? Absolutely! I don't know of any hood manufacturer that makes a rack that accommodates 6 fluorescent tubes. I've seen many, many 4-tube racks, but why settle for minimum? Maybe the manufacturer's know something we don't? Maybe that something is known as HO (High Output) and VHO (Very High Output). By using these types of "booster" tubes, wattages far in excess of NO/ Standard tubes, fluorescents make packing intensity into tighter and smaller spaces a reality.