Certainly frag tanks require much of the same equipment as traditional reef tanks such as a return pump, heater and possibly, a wavemaker. However, due to their reduced bioload and simpler setup there are some types of equipment that are generally regarded as not necessary. One of those pieces of equipment that is likely not necessary in a frag tank is a protein skimmer.
In a reef tank protein skimmers can perform a valuable function by removing dissolved organics from the water before they have time to negatively impact water quality. So why are they not needed in a frag tank? For starters, a frag tank tends to have a much lower bioload than a display reef tank. While a frag tank will generally have a couple of utilitarian fish and a small clean up crew it is unlikely that it will have the same stocking level as a traditonal tank. Similarly, a frag tank generally houses coral fragments as opposed to large colonies so there will be less waste produced by the corals as well. In fact, sometimes frag tanks can be “too clean” and not contain the low levels of nitrate and phosphate that are necessary for coral growth and colouration. Since protein skimmers reduce the levels of these compounds they may actually negatively impact coral health.
One of the appeals of a frag tank is their simplicity. Since they are often used to display corals at trade shows, the less equipment that has to be assembled and disassembled the better. While it is true that some hobbyists choose to run a protein skimmer in their frag tanks, especially larger tanks, they are probably not necessary in a smaller home frag tank and are not generally considered to be a required piece of equipment.