Astute salties out there will likely have observed that in some of our fish species profiles, we emphasize the need to keep the fish in a “mature” aquarium—meaning one that has been set up and established for at least six months or so (not one that has progressed beyond telling horrible knock-knock jokes or taunting you with “Neener, neener!” whenever a water-quality problem arises or a specimen dies).
Why does this matter to some fish? What difference does it make whether the tank is six weeks, six months, or six years old as long as it’s cycled and able to support life? Well, depending on the species, one or more of the following characteristics of mature tanks could be of vital importance:
Parameters have stabilized
Newly set up systems tend to be much more prone to fluctuating water parameters—plunging pH, drifting temperature, fickle specific gravity, etc.—than mature tanks are, especially for novice hobbyists trying to get the hang of maintaining their first marine aquarium. Even experienced hobbyists sometimes struggle to stabilize parameters in a new, unfamiliar system. While more rugged fish, such as damsels, may be able to shrug off such fluctuations, more delicate species might succumb to them.
The cycle is well established
The biological filter in a newly set up system is tenuous at best. It doesn’t take much—a little overfeeding or pushing the envelope with stocking—to overwhelm the resident population of nitrifying bacteria and cause an ammonia spike. Again, some very hardy fish might be able to endure this ammonia exposure while others might die very quickly.
Microfauna are on the menu
Quality live rock comes with a complement of tiny invertebrates that can serve as a smorgasbord for fish. In fact, some fish, such as the various dragonets and the Valenciennea spp. sleeper gobies, are prone to starving in captivity absent a steady supply of these microfauna. However, it takes time for these little inverts to establish healthy populations in a new aquarium. Add certain fish too soon, and they’ll quickly eat themselves out of house and home.
Algae is abundant
Hobbyists seem to be intent on eradicating it, but algae is a natural feature of coral reefs, and some fish, including many of the Centropyge spp. dwarf angels, must be able to graze on films of it throughout the day if they are to stay healthy. Of course, any herbivorous fish—tangs, rabbitfishes, etc.—will do best in a system with a decent crop of algae, which is usually a feature of mature systems (notwithstanding the sequential blooms of irksome algae that new tanks are prone to).
More detritus to munch
Species that include detritus in their diet, such as Ctenochaetus spp. tangs, which, incidentally, need a good crop of microalgae to graze on as well, are also best introduced to mature tanks, where this food source is more likely to be plentiful.