Blennies haven't enjoyed the popularity of other saltwater fish because they're rather colorless by comparison, but most reef-keepers believe that no saltwater tank is complete without at least one.
What they lack in color, these species of the family Blenniidae make up for in personality. They will dart along the bottom of the aquarium, propping up their elongated bodies on their modified pectoral fins, much like a seal. Some even have hair-like projections, called cirri, that sprout from the top of the head, making them even more comical as they jerk about on the bottom of your tank.
Most species of blennies that are kept in tanks come from tropical shallow waters near patch reefs, tidal pools and other such “cozy” shallow habitats along the coast. Some species live in cooler waters, such as the eel-like gunnel and wolffish - neither of which are common to keep. Some can move about on land. For example, the rockskipper Istiblennius zebra is a small Hawaiian blenny that lives along the shores and hops on land when it moves from one tidal pool to the next.
Blennies don't like to be disturbed and their behavior can even be described as “secretive,” but they are also curious and slightly aggressive at the same time. Thus, these little fish often seem a touch erratic, darting under a rock or overhang when danger approaches or when they are bothered. But should they become curious about something, such as a wandering crab, they will rush out to investigate.
Some of the Hawaiian and Pacific species are known to nip at hapless human swimmers. These little fish are bold feeders and will readily come out for a meal. Some species can be territorial and will defend the area they've chosen as their own. Do not keep them with very aggressive fish if you have a small tank since they may get into fights.
Some species of these scaleless fish are herbivores, some are carnivores. The meat-eating blennies prefer to feed on invertebrates such as worms or amphipods while the vegetarian blennies will be happy with a good supply of marine algae. It is important that you provide herbivorous blennies with enough food. One of the biggest mistakes in keeping herbivorous fish is underfeeding them. They are grazers and need a constant supply of greens.
Most blennies are hardy fish and have a tendency to be long-time residents once they have become acclimated to your tank. All blennies should have hiding places in various nooks and crannies in your aquarium, which can be created by adding rocks or shifting your artificial landscape to accommodate them. Lacking places to hide, some will bury themselves in the sand.
Types of Blennies
Of the 250 or so known blenny species, only a few are popular to keep. Several from the western Atlantic are becoming more available, but the most popular species that people keep in their tanks are the bicolor, redlip and golden blennies. These don't usually reach more than four inches in length when kept in an aquarium.