Gobies
Typically are hardy and do not harm invertebrates which makes them a good choice of fish for a reef tank.
The Black-ray Goby (Stonogobiops nematodes) is a species of marine goby and is a member of the family of High Fin Gobies. This type of goby is also sometimes attributed the common name "Shrimp Goby" or "Watchman Goby". This is due to the interesting symbiotic relationship that these gobies can form with Randall's pistol shrimp (Alpheus Randalli).
This fish is very docile and poses almost no threat to any other livestock inhabiting a typical marine aquarium. This passiveness makes it a perfect tankmate for delicate species like sea horses or pipefish. In fact, it is in reality quite shy, and when first introduced into an aquarium, may take up to several weeks before it is bold enough to leave its hiding place, or bolt hole. Whilst this fish can display aggression towards other tank inhabitants by opening its mouth and "yawning" at them, this is mostly show and the goby will quickly turn tail and hide if confronted.
The goby will spend most of its time hovering about two inches above its bolt hole, searching for scraps of food in the water column. If scared or startled, it will slowly retreat towards its hole. If the danger does not go away, it will dart inside at lightning speed.
Mated pairs of this fish are very rare and difficult to come by. Individual males may fight if placed in a tank smaller than about 50 gallons (~200 litres).
Elacatinus (formerly Gobisoma) is a genus of marine gobies, often known collectively as the neon gobies. Although only one species, E. oceanops, is technically the "neon goby," because of their extremely similar appearance other members of the genus are generally labeled neon gobies as well.
Neon gobies are very small, torpedo-shaped fish. Although sizes vary slightly by species, they are generally about 2.5 cm (1 inch) long. They have dark bodies with iridescent stripes (the color of which varies by species) running from the tip of the nose to the base of the caudal fin. Like all gobies, their dorsal fin is split in two, the anterior dorsal fin being rounded like that of a clownfish and the posterior dorsal fin being relatively flat. The anal fin lines up with the posterior dorsal fin and is of similar shape. The pectoral fins are nearly circular, and, like all other fins, transparent.
The yellow clown goby, Gobiodon okinawae, also known as the Okinawa goby or yellow coral goby, is a member of the goby family native to the western Pacific from southern Japan to the southern reaches of the Great Barrier Reef. As the name implies, they are bright yellow in color, save for a whitish patch on each cheek. Equally bright as their coloration is their personality; they are valued additions to reef aquaria, with a reputation for being friendly and entertaining.
G. okinawae have a generally fusiform shape with seven dorsal spines, ten soft dorsal rays, one anal spine, and nine anal soft rays. At maturity they can reach a length of 3.5 cm.
Another species, Gobiodon citrinus, the citrus goby, has a similar appearance to Gobiodon okinawae. G. okinawae, however, lacks the requisite blue and white facial markings of G. citrinus.
These fish are not generally aggressive, though they can actively protect their territory against their own kind. Their primary defense against predators is a poisonous, bitter mucus on their skin that makes them unpalatable.