I would suggest nothing small than a 2 gallon tank. I understand it is for work so it should be small. But a 2 gallon will let you add a small filter, a heater (offices generally go cold at night with no people and lights off) and add some live or fake plants. A simple bubble/sponge filter will do nicely. A small desklamp with a fluorescent bulb will provide good daily light. Change the tank water 50%, maybe twice a week. Adding a low light easy plant like java fern will help keep the water quality good. If you are away fromt he office for more than a few days at a time, premeasure some food out in little packs and have some one dump a single pack in once a day at most.