that distinguishes a nano reef from a regular reef is somewhat ill-defined. Some claim that anything less than 180 litres (40 Imperial gallons / 48 US gallons) would qualify. But 140 litres (30 Imperial gallons / 37 US gallons) seems to be the generally accepted limit. Nano reefs have become quite popular in recent years among fish keeping hobbyists, primarily because of their smaller size, maintainability, and the possibility of lower costs. The burgeoning interest in this niche of marine aquarium science has fostered several notable contributions ranging from specific consumer products such as specialized aquarium filters, compact high intensity lighting systems and smaller circulation pumps. Such equipment allows the aquarist to maintain an environment wherein many marine organisms are capable of thriving. The first reference to these small reef aquariums was made in 1989 by Albert J. Thiel in his book Small Reef Aquarium Basics. Nano reefs are very commonly sold as complete kits which contain the tank, stand, power compact T5, T8, PL lamps or metal halide lighting, protein skimmer, UV steriliser, 3 or more stage filtration, a heater and a water pump or powerhead. However, many nano reefkeepers decide to upgrade their aquariums with better quality equipment such as a more powerful protein skimmer or lighting. Pico reefs Another term gaining popularity is pico reef, which is used to refer to the smallest of nano reef aquariums. Most online forum polls set the range of approximately 10 litres (2.5 gallons) and below as pico reefs. These tiny tanks require even more diligence with regard to water changes and attention to water chemistry because the small water volume provides little room for error. Care must be exercised when stocking these tiny tanks because too many inhabitants can easily overload the tank's ability to process wastes effectively. For the smallest of pico reefs, even the presence of a single fish is discouraged. Pico reefs often consist of live rock, hardy corals, and small invertebrates such as hermit crabs and marine snails. The keeping of pico reef aquariums has tested the extent of allelopathy, the chemical and physical means by which corals compete for space. Before the advent of these concentrated environments, it was thought to be impossible for corals of even a few mixed genera to occupy such a small shared water volume. Challenges associated with small reef aquariums Due to the small water volume, fluctuations of water quality