What Is a Reef Tank?


Reef tanks are the most challenging of aquaria.  One reason is that a reef tank unlike all other types of aquaria must keep a careful ecological balance among its inhabitants to maintain the proper water quality and control predation within certain boundaries.

The cornerstone of this balanced ecosystem is rock that is obtained from the ocean that harbors a myriad of life forms, which form the lower portions of this ecosystem.  This rock is called ‘live rock’ based on the fact that it contains various living organisms on and within its structure.  These organisms range from bacteria to sponges, fan worms, tunicates, macro algae, snails, crabs, shrimp, soft corals or even hard corals.  Pretty much anything that grows on a rock in the ocean can be on the live rock when it gets introduced into the reef tank.

Some of this life cannot stand the transition between ocean and tank and dies off in a process called ‘curing’ the live rock.  There are also things that may come with the live rock that are not desirable, such as certain forms of algae, predatory shrimp or crabs, etc.  The important thing is that live rock establishes a diversity of life forms within the tank that would be impossible to achieve through only intentional introduction of organisms.  The exciting part of this is that you never quite know what you may find in your tank.

Recently synthetic live rock (man-made rock that has spent some time in fish tanks) has been put on the market, but using it alone without seeding from  natural live rock has not been very successful from my own experience.  While it contains some of the beneficial bacteria of live rock, it does not contain any of the higher life forms that are important to the diversity of the ecosystem.

Live Rock

Live Rock

Another element that defines a reef tank is the inclusion of corals or other reef dwelling invertebrates such as anemones.  The first reaction many people have to a reef tank is “where are all the fish?”.   Invertebrates take top billing in a reef tank with fish taking second billing.  The reason is two-fold.  One is that the fish population must be maintained within certain boundaries to keep the ecological balance.  The other is that the corals or other invertebrates are really the main stars of this type of tank.  If a person desires the highest possible fish density, then a fish only (FO) tank or a fish only with live rock (FOWLR) tank is the best approach to take.  These tanks can be crammed (relatively speaking) with fish and mechanical filters and large water changes employed to manage the waste byproducts produced by the fish to a level that keeps it within tolerance levels of the fish.  This tolerance level is much higher in fish than what corals can tolerate, so this approach is not viable when corals are involved.  Another consideration is that many of the popular saltwater fish consider the invertebrates in a reef tank as food, so fish selection for reef tanks is more limited than in FO tanks.

The inclusion of delicate invertebrates, many of which are photosynthetic, mandates different equipment be deployed on the tank.  More powerful lighting is required for most coral for instance than what is required in an FO tank.  Ancillary equipment such as protein skimmers may be deployed to maintain a high water quality and generally reef tanks require supplementation of elements such as calcium as corals, clams and similar invertebrates consume various elements to grow skeletons and build shells.

Lastly, a reef tank ‘demands’ attention from the aquarist.  Exactly how much attention the tank demands depends on the sort of reef tank that is setup.  This is covered in more detail in the ‘Types of Reef Tanks’ section.  The important take-away here is that a reef tank is not something that you setup once and then occasionally throw some flake food in.  It requires attention and occasional fiddling to keep things running smoothly.  Even then, things will still go wrong that require quick corrective action before things get out of control.