The number after the T indicates the diameter of the bulb, a higher number means a larger diameter. T-12 is the typical fluorescent bulb. A T-5 bulb is less than half the diameter of a T-12. Within the different sizes, bulbs are manufactured to handle different levels of wattage to get different levels of output (VHO, HO, and normal output (NO)). In aquarist lingo, “VHO” always means T-12 VHO, “NO” usually means T-12 Normal Output, and “T-5” usually means T-5 HO. I’ve never heard of a T-5 VHO or a T-12 HO.

Now to make things confusing, aquarist often use Icecap and Workhorse ballasts designed for VHO bulbs to run T-5 bulbs. Doing this “overdrives” the T-5 bulb, running it at a higher wattage than it was designed for. A 4 foot 54 watt HO T-5 bulb will run at 80 watts when it is overdriven with a VHO ballast. The same ballast will run a 4 foot VHO bulb at 110 watts. Since it runs at a higher wattage the VHO has more output than the T-5 bulb, however, the small diameter of the T-5 bulb allows it to be fitted with a SLR (single light reflector) that very efficiently directs the bulb output into the aquarium. Thus the T-5 with a reflector can with fewer watts rival (surpass?) the usable output of a T-12 VHO. Most if not all VHO bulbs have internal reflectors, but these are not as efficient at directing light into the aquarium as a T-5 with a SLR.

Two essential parts of getting the most out of the T-5s are 1) to have a properly designed SLR (Icecap and Tek II lead the pack) and 2) having cooling fans directed at the bulbs when overdriving T5s. Without proper cooling, the output and life of an overdriven T5 bulb is reduced significantly.