Marine Aquarium Basics, Part 10: Stocking Invertebrates

Author: Philip Hunt

In the 10th installment of his series on the fundamentals of a saltwater aquarium, an expert marine fishkeeper explains how to choose the best invertebrates for your tank.

Choosing Tankmates

In the last installment of this series, we looked at selecting fishes for marine aquariums. This month, we look at the more complex topic of stocking the aquarium with invertebrates. This is more complicated because of the sheer diversity of invertebrates that can be kept. Potential invertebrates for the aquarium range across taxonomic phyla from Cnidaria (corals and anemones) to Arthropoda (crustaceans, for example), Annelida (segmented worms), Mollusca (clams, snails, and cephalopods, among others), and more.

While it’s impossible in a relatively short article to cover every aspect of stocking all kinds of invertebrates, as with last month’s piece on fishes, we’ll look at general principles and focus on those creatures that are of most interest. As corals and their relatives are the major invertebrate inhabitants of most reef aquariums, I’ll start with them.

Corals and Other Cnidarians

Deciding which of the huge range of available corals to keep is made easier by the fact that your choice is limited by the manner in which your aquarium is designed and maintained. If, for example, your lighting is not super bright, there’s little point in trying to keep corals that need intense illumination, or if you maintain very low nutrient levels in your aquarium, chances are that mushroom anemones and some large-polyp stony (LPS) corals will not thrive in it. As discussed earlier in this series, wise reef aquarium keepers plan their systems around the intended inhabitants, particularly corals. If you haven’t done this, you should at least research which corals are likely to thrive under your tank’s conditions before you go to the store.

Wild, Maricultured, or Captive-Grown?

In addition to the choice of coral species on offer, increasingly you’ll find there are options with respect to their sources and how they were grown. There are basically three ways to go: wild-collected, maricultured, or captive-grown.

Wild-collected corals are taken from a wild source and imported directly. Maricultured corals are

propagated by taking fragments of wild colonies, fixing them to a substrate, and growing them out in the ocean, usually not far from the wild source. Captive-grown corals originate as fragments from wild colonies, but unlike maricultured colonies, they are grown out in aquariums.

Some corals are mainly, or exclusively, sold as wild-collected colonies because of the difficulty or impossibility of propagating these species. Other species, particularly small-polyp stony (SPS) corals, may be available in all three forms.

If there’s a choice, it’s generally best to go with captive-grown specimens or frags. Such corals are generally better adapted to aquarium life than maricultured or wild colonies—they have already been through a process of selection and are clearly able to do well in the aquarium. Some captive-grown colonies may be the result of several generations of fragging, such that the colony you buy may be grown from a fragment of a colony that was grown from a fragment of a colony, and so on. Also, captive-grown corals are often cheaper, usually because you’re buying a frag or small colony.


The downside of stocking your aquarium with captive-grown corals is that it takes longer to get large, well-established colonies when starting from small specimens. There is also one caveat: Although captive-grown colonies and frags are well-adapted to aquarium life, that doesn’t mean they will grow under conditions that would be unsuitable for the species. An Acropora frag, even one that is 10 generations removed from the wild, still needs strong water movement, low nutrients, and intense light.


Before moving on from captive-grown corals, it’s worth noting that you should be a little careful when buying frags of some LPS corals, such as faviid corals (Favia, Favites, and Platygyra species, among others) and mussid corals such as Acanthastrea. Wild-collected colonies are sometimes broken or sawn up, the pieces allowed to heal and then sold as captive-grown frags. If the frags are healed and obviously growing well, this is fine, but freshly cut pieces may not always survive, and if they do, they’ll take longer to grow into full-size colonies.


Maricultured colonies, being grown in the ocean and thus used to wild conditions, tend to be similar in their aquarium hardiness to wild colonies but more easily available and less expensive than their wild-collected counterparts. Mariculture allows large numbers of specimens to be propagated from brightly colored colonies (which may be rare and difficult for collectors to find). Maricultured colonies are usually larger than captive-grown ones, so using them can get your aquarium established faster. Buying maricultured corals has some other advantages in that it helps support local economies where corals are collected, and this can have a beneficial effect on reef conservation, giving people alternatives to destructive fishing practices as a way to make a living.

Wild-collected colonies represent the traditional source of aquarium corals, and for some species, such as LPS corals like Trachyphyllia and Scolymia, there is really no alternative. Also, wild-collected colonies are perhaps the riskiest corals to buy. This is particularly the case for many SPS corals, which are often more difficult to acclimate to aquarium conditions than most LPS species. On the other hand, many familiar and widely kept soft and stony corals proved their worth in the early days of reef aquariums, and at that time, just about every coral was wild-collected.

Choosing Healthy Corals

Having decided which corals to buy, and from what source, how do you know that what you’re buying is healthy? This can sometimes be tricky in a store, as corals may not be held under ideal conditions for every species (they are, after all, only likely to be there for a short time), which may make their health more difficult to judge. There are a few obvious things to look out for that should ring alarm bells, however.

Examples of obvious disease symptoms include tissue recession exposing areas of white skeleton (particularly in SPS corals); cyanobacterial overgrowth on the coral; or brown, jelly-like material covering part of the soft tissue.


Also, watch for evidence of bleaching, particularly with LPS corals (often faviids). For example, you may see specimens that look white with very obvious, usually green, fluorescent pigments—what you’d expect to be a brown-and-green coral is a white-and-green one. These are bleached, meaning they have expelled their zooxanthellae. They might recover (as usually happens when corals bleach in the aquarium), but the additional stress of shipping might be too much for them.


Physical damage may precede infections and other problems. If you are going to buy a coral with obvious signs of damage (areas of bare skeleton, perhaps), be sure the tissue has healed and shows a firm, clear edge. This is generally less significant for soft corals than stony corals. Some corals may have been chipped off larger colonies during collection, so look carefully at the edges of the colony for a good, firm, healthy-looking margin.

Watch out for hitchhikers. You may find Aiptasia or majano anemones growing on the same rock as your coral or maybe attached to the coral’s skeleton somewhere. If you buy the coral, be prepared to remove such “reef weeds” promptly.

Tissue and Polyp Expansion

In terms of positive signs, the big thing to look out for is good expansion of tissues and polyps. This, however, isn’t always straightforward to assess. As mentioned, if a coral is not held under optimum conditions for that species, the coral may not be fully expanded even when it is perfectly healthy. Some species retract their polyps regularly (especially when adjusting to new aquariums). Many soft corals do this, as do some stony corals such as Goniopora and Porites species. Other species (some LPS corals, for example) may have a pattern of expansion and contraction over the course of the day, so how well expanded they are may depend on when you see them.

This is quite a list of things to consider, but the main determinants of success are choosing the right species for your aquarium conditions, selecting corals from appropriate sources, and avoiding specimens showing any of the negative points noted above.


A wide range of crustaceans can be kept in aquariums, most commonly a variety of shrimps and hermit crabs, but also some crabs and small lobsters. Choosing which species to keep is important. While some can make great inhabitants for a standard reef aquarium with a wide range of other inhabitants, others can eat invertebrates or even fishes. Other species may be very reclusive in large and busy aquariums, and will both do better and be more visible in nano tanks or systems designed around their specific habitat needs. This is yet another case where doing your research before buying is very important.

Choosing healthy crustaceans is relatively straightforward. Along with observing their general demeanor (even if not actively moving around, they should be in healthy postures, not sitting listlessly on the bottom), there is one specific sign to look for. In healthy crustaceans, the mouthparts are in constant, rapid movement.

Crustaceans require special attention when they are being added to aquariums. Many of them are very sensitive to changes in salinity and water quality, so they should be acclimated to new aquariums very slowly, as described later.


Two main categories of mollusks are commonly kept in marine aquariums: gastropods of various kinds, including snails and cowries, and tridacnid clams. Reef aquarium hobbyists often keep a variety of snails in order to keep algae under control. The right snails can make a real contribution to preventing algae problems, but some species are much better at this than others.


Some snails can present a challenge when it comes to choosing healthy specimens. While it’s good to be able to watch snails in the store tank and pick out those that are actively moving around and feeding from rocks or the tank glass, some species are not particularly active during the day, so a lack of movement isn’t necessarily a bad sign in these species. One good indicator of health is for the snails to be firmly holding on to the glass or rocks—even when not active, healthy snails attach themselves firmly to the substrate.

Tridacnid Clams

Tridacnid clams are among the most beautiful creatures you can keep in a reef aquarium. The choice of species depends mainly on two factors: the size of your aquarium and the intensity of the lighting. Among the commonly available clams, Tridacna crocea and T. maxima (particularly the former) are the best for smaller aquariums, as they don’t grow as large as the other species. Both of these species require very bright light, however, and T. maxima seems to be more difficult to keep than others in the family. T. squamosa and T. derasa grow larger and need bigger tanks, but T. derasa doesn’t require quite such intense lighting.

Choosing healthy clams is fairly easy. Look for individuals with well-expanded mantles that are colored all over with no pale or bleached areas. Healthy clams (especially when newly imported) will usually close their shells when a shadow passes over them, whether from a fish or a hand—although they often become less reactive after a long residence in the aquarium, not because of ill health, but due to becoming accustomed to the absence of predators.


One particular thing to watch out for in clams is gaping. A clam in this condition will have its shell wide open, but the mantle will appear to be stretched between the lips of the two halves of the shell, and the inhalant siphon will appear to be pulled open. Clams in this condition are usually dying. Also, check the underside of the clam for any evidence of small snails. Clams may be attacked by parasitic snails (about the size of a grain of rice) that are typically found underneath the shells. The snails can be removed by gentle scrubbing with a toothbrush, and some fish will eat them (such as various Pseudocheilinus spp. wrasses), but it is obviously better if the clam is free from parasites when you buy it.

Many mollusks are very sensitive to changes in salinity, so a slow equilibration process (see below) is essential when adding them to the aquarium.


While a wide range of different echinoderms are offered for sale, few are suitable for the general reef aquarium, as most require specialized care. Some starfishes are known to eat sessile invertebrates, and some tend to slowly starve to death in aquariums (in many cases, their preferred foods are unknown), as do basket stars and sea apples.

Many sea cucumbers meet a similar fate, and in addition, when irritated or injured, they can release a potent toxin (holothurin) that can wipe out all the animals in the aquarium. Some can also discharge a mass of sticky filaments, called Cuvierian tubules, from their bodies in an attempt to entangle a tormentor.

Most sea urchins, while being potentially useful algae grazers, may pierce coral tissues (or hobbyists’ fingers!) with their spines, and are so strong that they can dislodge or break corals as they move around the aquarium.


The only echinoderms that are reliably good (and very useful) in the typical reef tank are serpent stars and brittle stars. These are interesting, if sometimes secretive, creatures that make great additions to the aquarium’s janitorial crew, as they work their way around the tank, often after dark, clearing up uneaten food.

Avoid Green Brittle Stars

There is one species to avoid in tanks housing smaller fishes: the green brittle star (Ophiarachna incrassata), which is a notorious predator of fishes. This is a very distinctive creature, a dull green in color with thick, muscular arms that have short black and yellow spines. It grows to a spectacular 50 cm (20 inches) across. It’s quite an interesting beast but can only be trusted with large fishes. Other serpent stars, e.g., Ophioderma species, and brittle stars e.g., Ophiocoma species, make very good, robust, and long-lived reef aquarium inhabitants.

Like crustaceans and mollusks, echinoderms can be very sensitive to changes in salinity and so require slow acclimation to the aquarium.

Introducing Invertebrates to the Aquarium

Because of the diverse physiology of invertebrates, optimum methods of introducing them into the aquarium vary among different types of creature. Some invertebrates, such as most corals, can simply be acclimated to aquarium temperature by floating the shipping bag in the tank for 20 minutes or so and then placing the coral straight into the aquarium. Some enthusiasts advocate an even more radical approach, which is to take the coral straight from the shipping bag and place it into the tank with no equilibration. Surprisingly enough, this seems to work well in most cases, and it has the advantage of getting the coral out of the shipping water (often full of mucus and waste products) quickly.

In contrast, many crustaceans, mollusks, and echinoderms are very sensitive to changes in salinity, and so require a slow acclimation method when being added to the aquarium. A good method to use is to transfer the new creature and its shipping water to a container, then set up a slow siphon using a narrow-bore pipe (such as airline tubing) to add the tank water, using a clamp or valve to control the flow.

Allow tank water to drip into the container such that it takes about an hour to add a volume equal to the shipping water, continue for another 90 minutes or so, then transfer the animal to the aquarium. Alternatively, you can add small amounts of water every few minutes, aiming to achieve roughly the same rate of water addition. Although this is a time-consuming process, it should ensure your new acquisitions get off to a good start in your aquarium.

Stocking Levels for Invertebrates

Unlike fishes, for which it is both critical and relatively straightforward to determine how many can be kept in a given aquarium, establishing how many invertebrates a tank will hold is both more complicated and, in a sense, less important. Sessile invertebrates, such as corals and clams, place very little biological load on the aquarium system; indeed, clams in particular can take up nitrogenous wastes from the water, acting as biological filters in themselves. Limits to the numbers of sessile invertebrates are dictated more by the available space to fit them in than by rules based on the volume of the aquarium.


In the wild, corals compete with each other for space on the reef and use an arsenal of different weapons to overcome their neighbors. Many soft corals produce toxic substances that inhibit the growth of other species, particularly stony corals. These substances can accumulate in aquarium water to the same effect, making it difficult to grow some stony coral species in aquariums that are stocked with soft corals such as Sarcophyton and Sinularia species.

Skimming and chemical filtration can help eliminate these toxic substances, but it can be difficult to maintain certain mixes of corals within the aquarium. These types of soft corals can be kept at high densities, however, as they do not harm each other. The same is also true of many other soft corals, most zoanthids, and mushroom anemones (although some of the latter do appear to be capable of stinging neighboring organisms).

Stony corals have different ways of clearing living space around them. Their usual tactics are simple and violent: They sting their neighbors using tentacles or even try to digest them using mesenterial filaments extruded from their gastric cavities.

Creating Space

The upshot of all this is that when placing stony corals in the aquarium, you need to leave some space around them, partly to provide for growth (especially with fast-growing SPS corals), but also to allow for the production of extra-long stinging tentacles. These so-called sweeper tentacles, which appear to be distinct from normal feeding tentacles, are most prominent in LPS corals and often produced at night. Some species can produce sweeper tentacles that are several inches long.


Stony corals vary greatly in their aggressiveness. Some are notorious stingers of neighboring invertebrates (bubble corals, Euphyllia, and Galaxea species, for example), whereas others, such as Trachyphyllia, Porites, and Montipora, are more passive.

Snails and Hermit Crabs

Two specific types of invertebrates that may be subject to issues with population levels in the aquarium are snails and small hermit crabs. Both are usually stocked in order to control the growth of algae, and you can find many recommendations for keeping large numbers of them for this purpose. Recommendations of one snail or crab per gallon of tank capacity were very common a few years ago, although they’re less often seen now.

If too many snails are added to the aquarium, they will rapidly deplete supplies of the algae that they eat (some are quite fussy in this respect), then most of them will starve to death. The best approach when stocking snails is to add a small number at first, observe their effects on the algae in the tank, and cautiously add more if there is still enough algae to support them. It’s also important to stock snails of an appropriate size for the aquarium. While Astraea species tend to stay small, and so can be kept even in nano tanks, others, such as Trochus and Tectus species, can get quite large and need a lot of algae to sustain them.

Small hermit crabs eat a wider range of foods and are less likely than snails to starve if there are too many specimens for the available supply of algae. They may, however, be more likely to fight under these circumstances and may indulge in other bad behaviors, such as pulling snails out of their shells (sometimes the snails get eaten, but sometimes the crabs are only interested in the shells). Hungry hermits may also start eating other potential foods, sometimes damaging corals in the process.

As with snails, it’s best to exercise caution when stocking hermits, but if you do find yourself with a large population and not enough algae to go around, providing a supply of suitable shells and keeping the crabs well fed can help to prevent problems.

In the next part of the series, we’ll look at aquarium maintenance.