Stocking Your Marine Aquarium

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Niger Trigger: The fine art of creating communities. Scott Michael

How to Mix & Match Fishes in A Balanced Marine Community

Most potential dog owners—or at least the prudent ones—spend some time researching the breed or breeds they are thinking about buying. If you’ve ever owned or paged through a book on canine puppies and breeds, you know they usually have a sizeable chapter on selecting the “right dog” for your specific lifestyle.

I only wish that more marine fishkeepers were as thoughtful when choosing new species for their aquariums. Given the right conditions, a coral reef fish can easily live as long as a puppy, perhaps even longer. With reasonable care, many can live more than 10 years, and 20 years is not unheard of. If you are good at picking the right fish, it can become a long-term member of your household. Doing a bit of initial research before bringing home a new fish can prevent many of marine aquarium keeping’s most predictable headaches and heartbreaks.

A Sea of Choices

Walking into a fish store with a large marine fish section can be mind-boggling if not downright intimidating—there is so much variety! Not only do these fishes display an amazing array of body shapes and color schemes, many may differ in their behaviors, their natural ecology, and their captive requirements.

For example, some of the fishes available to us are highly territorial or so predatory in the wild that they will wreak havoc in a tank full of smaller or milder-mannered species. Pick a species with these instincts and you may see the rest of your fish collection battered or even eaten by the new tankmate.

Then there are fish species so shy that you’ll be lucky to see them once they are introduced to a tank full of live rock and myriad hiding places. Choose unwisely and these furtive or passive fish may hide constantly and fade slowly away from starvation and stress.

Feeding is a highlight of the day when keeping a marine aquarium, but some fishes require two or even three or more meals daily to stay healthy. Others have very specific diets that can be time-consuming, expensive or impossible to provide. Some species need lots of swimming room, while others must have a deep layer of sand to burrow into. Finally, there are also marine fishes available at your local pet store that are destined to grow huge and overwhelm any home aquarium.

To create a balanced, beautiful, interesting marine system, the thinking aquarist will need to be aware of the profile and husbandry needs of each fish before making a purchase and adding a new fish to his or her tank.

Aquarium Fish Species Profiler

Questions to ask before acquiring a new fish or invertebrate:

  1. How large does this species get? When it reaches adulthood, will it fit into the tank you intend to set up?
  2. How aggressive is it? What sort of tankmates might it harm?
  3. What does it eat? Does it have specific dietary requirements that will be difficult to meet or does it have a generalized diet?
  4. Is it piscivorous (a fish eater) or will it graze on the choice invertebrates you want to keep, such as ornamental shrimp, corals or clams?
  5. What are its habitat preferences? Is it a fish that demands lots of space? Does it have any special aquascaping requirements? If it is a coral, does it need high-intensity lighting?
  6. Is it susceptible to disease?
  7. Is it considered easy or difficult to keep? Do you have the skills (and resources) to be sure it has a chance to thrive?

Planning Your Fish Community

So where do you start in planning a fish community and finding out about species-specific traits and care requirements? The first step is to make a list of the species that you are interested in keeping—ideally, you do this even before you purchase the aquarium or before you start to re-populate an existing tank. Look through this and other reference books with good profiles of marine aquarium species or take a notebook to your favorite fish store and jot down the names of all the species that you find interesting. I would cast a wide net. Write down any fish that strikes your fancy. As you build your list, put an asterisk by those species that you want the most. I call these “the must-have species.”

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Now the real research begins. For each species, you need to fill in a few vital blanks. Start with books, online aquarium news groups, and the staff at reputable local fish stores. I have provided a short list of questions that you should try to answer about each species in the Marine Species Profiler on page 19. You will also want to get some idea how difficult it will be to get the fish on your list and how much they might cost.

In narrowing your choices, you may find that you want to build a captive fish community around one of your “must-have species.” For example, if one of the asterisked fish is very aggressive, you may have to limit your fish collection to other aggressive fishes or larger fishes that your pugnacious centerpiece fish is more likely to ignore. For example, if your key species happens to be a grouper or lionfish, you will have to choose tankmates that are too large for it to swallow.

If, on the other hand, your aquarium world must revolve around a beautiful group of Flasher Wrasses, small and/or peaceful tankmates will be required. Some species are either so aggressive or so easily intimidated that they deserve to have a whole aquarium dedicated to their special needs. For most newcomers and intermediate fishkeepers, a mix of compatible species makes the most sense.

Habitat Considerations

One benefit to having a stocking plan is that your final fish selections may influence the way you set up and aquascape the tank. It may also have some bearing on the size and shape of the tank you purchase. For example, if I decide I cannot live without one of the large, active species, such as the French Angelfish, I will need to provide enough swimming room to accommodate its size and need to roam. This may mean keeping the décor to a minimum, selecting a tank with more surface area or a larger tank than originally planned. If one of the most important fish on my list likes to spend its time on or burrowing in the substrate, I will need to have some open sand bottom so it can do what comes naturally. If I keep species with pugnacious tendencies, I need to provide plenty of hiding places—rocky caves and niches—for all of my charges.

After doing the research, your list has been whittled down considerably. Now, if you are planning a new system, put the species in order from least aggressive to most aggressive. This is very important, because you will want to add the least aggressive fishes to your tank first, and the most pugnacious species last.

In ranking fishes by disposition and relative aggressiveness, a number of variables come into play: how a fish acts will depend on the size of the tank, the size of the fish compared to its tankmates, the number of hiding places available, what other species are being kept, and even the number of times a fish is fed a day will have some impact on how it behaves. There will also be that freak individual that breaks all the rules set for its species.

Remember, fish don’t read the books­—what makes them interesting is that they don’t always do what the literature says they will. The disposition ranking is only a rough guide for a particular species.

Stocking Density

There are a number of variables that will determine your tank’s fish-carrying capacity. Using live rock, protein skimmers, and other modern filtration methods, we can more efficiently keep nitrogenous waste products at bay. More often than not, behavioral factors will limit the number of fish in a tank. Space and hiding places are usually the limiting factor today in many fish-only aquariums.

When you overcrowd your tank, aggression (and the resulting stress and disease outbreaks) becomes a more acute problem than the carrying capacity of the biological filters. Of course, an overcrowded aquarium can lead to deteriorating water quality (declining pH and the build-up of dissolved organic wastes and nitrate) as well. All of these factors can make fish more susceptible to disease, listlessness, color loss and other ailments. Therefore, it is always better to have one or two fish too few, with a small but healthy margin for error.

Predicting the appropriate stocking limit of a tank can be tricky and is often more art than science. Many equations have been proposed to enable you to find the carrying capacity of a tank’s biological filtration. But these “inch-per-gallon” equations have inherent problems. First of all, the mass of the fish, not the length will determine the amount of waste that it produces. A hefty 6-inch grouper is going to produce more waste than a pencil-thin 6-inch moray.

Secondly, the metabolic rate also varies from one species to the next. Active wrasses and anthias, for example, are going to consume more food and produce more waste than a slow-moving comet. Food consumption, and associated waste production, is also likely to be greater for a younger fish than an adult.

As we develop our fish communities, we must also allow room for growth. Under favorable conditions, your reef fish will grow quickly. A Koran Angelfish may be just two inches long when you acquire it, but it can easily triple its size in a year’s time. Unless you plan on buying a larger tank to accommodate growing fish, you need to put together your community for adult fish, not the juveniles that most of us start with. Yes, the tank may look somewhat empty initially, but the fish will grow to fill the space with astonishing speed.

Adult Size Factors

Predicting the maximum size your fish will reach in captivity can be difficult. A Queen Angelfish, for example, can reach a maximum of 18 inches in the wild, but few attain this top size. Larger species more often than not only attain 75 to 90 percent of the maximum length recorded, while smaller fishes (grammas, dottybacks, fairy wrasses, flasher wrasses, gobies, blennies) regularly come close or reach the proposed maximum sizes listed in the reference tables.

So—how many fish can you realistically expect to keep in your 75-gallon tank? Here are some rules from other published “experts:” one inch of fish per 2-5 gallons of tank; 2 gallons per medium-sized fish (2 to 3 inches or less in length); six to eight fish, no bigger than 3 or 4 inches, for every 27 gallons of water; 1 medium-sized fish per 10 gallons of water. According to the most conservative of these equations, you could keep seven or eight, three-inch fish in your 75 gallon tank, while the most liberal allowances suggest that you could go with over 30 fish that are two to three-inches in length.

Bewildered? I suggest a number somewhere toward the lower estimate, perhaps 12 to 14 small to medium-sized fish, or about two fish per each ten gallons of tank capacity. Once again, remember that tank size and biological filtration are not the only things to consider. You also need to think of behavioral factors and the types of fish going into the aquarium. See pages 28-37 for some balanced fish communities that can fit into a range of tank sizes. If you do regular maintenance, have good biological filtration, and aquascape as suggested, these communities can be successfully maintained with minimal risk of serious aggression between tankmates or losses due to predatory behaviors.

Pacing Your Population Plan

To my mind, the most important thing when stocking your tank is to go at it slowly and enjoy the process. It should be at least six months before you reach your aquarium’s carrying capacity.

The worst mistake I see is trying to reach the population apex as fast as possible. During a stint in the aquarium retail business, I witnessed attempts to do this on a number of occasions—usually the “grand opening” of a new home or business. Disaster (for the living specimens involved, as well as the owners’ pocketbooks) was the result in almost every case. There is a saying among veteran aquarists: “Nothing good ever happens fast in a marine aquarium.”

Those who scuba dive will also be very familiar with the mantra: “Plan your dive and follow your plan.” The same applies in stocking a marine aquarium. Have a concept for your aquarium and stick to it. Inevitably, a great fish will appear in a local shop and unexpectedly catch your eye, but I urge you to ask the basic questions and learn about its behavior and husbandry requirements before bringing it home. Somehow each new fish must fit with the overall scheme for the aquarium community you are creating.

If you find that the species in question has a tendency to be more boisterous, it should be added into the tank only after the less aggressive species have had a chance to settle in. Because we need to take our time when adding fish to our aquariums, you may have to wait to purchase this new fish until more of the species on your list are added to the tank. Other opportunities will come along.

Be aware that spontaneous, uninformed purchases can scuttle the best of stocking plans and can come back to haunt you. “I HAD to have that triggerfish,” is one recent admission I heard from a well-heeled hobbyist. “The next morning I found he had eaten my whole collection of Tridacna clams.” Fatter by hundreds of dollars worth of clam flesh, the trigger had to be returned.

Know when to stop. When you reach your tank’s carry-capacity you don’t purchase more fish. Restraint is not easy, and I know this from personal experience. Buying a new fish is undeniably exciting, but self-discipline is important when it comes to buying any live animal. Succumbing to the temptation to add just one more fish to a fully stocked aquarium can jeopardize everything you have accomplished. Always populate a tank with the future growth of the species you purchase in mind.

Of course, if your fish are outgrowing their space or you have absolutely no room for new species, you have a great excuse to start thinking of setting up a new, larger system.

Aggression—Imported from the Reef

Behavioral stress, usually caused by one or more irascible tankmates, can cause long-term problems in a fish community. Many reef fishes naturally stake out and defend only a limited territory in the wild, and they are typically not indiscriminately aggressive. They display antagonism towards members of their own species and other fishes that compete with them for shelter or food and they recognize competitors by their shape, color and behavior.

When a fish is collected for the aquarium trade, it brings all this behavioral baggage along and into its new aquarium home. Our task as marine fishkeepers is to manage or curb these aggressive tendencies. We do this by carefully selecting the types of fish that we put together, by introducing the fish into the tank in a logical order and by making sure there are enough necessary resources—most importantly, hiding places and food—to go around.

There are a number of factors that determine how one fish may behave toward another species in captivity. There are species specific differences. That is, some species have a greater propensity to be aggressive than others. There is the case of prior residence. If an individual has been in a tank for a while, it is more likely to be aggressive to a newcomer. A fish is also more likely to behave aggressively if there is a lack of space and shelter. If these valuable resources are in short supply, tank occupants will fight to acquire and keep them. Finally you must consider the physical and behavioral characteristics of the potential aggressor's tankmates.

As mentioned above, reef fish do not indiscriminately attack other fish, but recognize competitors. Studies have shown the most important recognition cue is body shape, with individuals displaying more antagonism toward species with similar body plans. Body shape, to some degree, is an indicator of diet. The majority of herbivores are deeper-bodied and laterally compressed, while most predators are more elongate. Therefore, it makes sense for a fish to chase away species with the same form, because they probably also have a similar diet.

Color is also an important recognition cue. A study carried out on captive butterflyfishes indicated that species with similar color patterns were more aggressive towards one another than they were to those that differed chromatically.

Finally, the feeding behavior of a species can make it susceptible to attack. At least one damselfish identifies competitors by how they feed. If an approaching fish is grazing along in a characteristic herbivore manner, nipping at the substrate, a resident damsel will attack it and drive it from its territory.

Scott W. Michael
Reference: The 101 Best Saltwater Fishes