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Fish Selection and Stocking GuideFish Selection & Stocking Guide

In a perfect world, stocking an aquarium with fish would be as easy as picking out the prettiest or most interesting specimens, taking them home, and plopping them in a tank full of water. We’d never have to worry about our fish getting sick, being infested with parasites, refusing to feed, or squabbling with tankmates. But in the real world, selecting and stocking a healthy, compatible community of fish isn’t so simple—and it doesn’t happen by accident.

After reading this Fish Selection and Stocking Guide, you’ll understand what to look for and what to avoid when choosing fish, how to select species that will coexist in relative peace, how to acclimate new specimens to your aquarium, and how to avoid introducing nasty diseases or parasites to your tank along with the fish. Armed with this knowledge, you’ll be ready to turn your vision of a lovely, tranquil community of freshwater or marine fish into reality.


Choosing WiselyFish Selection

When you visit your local fish store, the dazzling array of specimens can really make your head spin. They’re all just so beautiful! But proceed with caution. Buying fish on impulse because they’re pretty or exotic-looking is a recipe for failure. When selecting fish, it’s critical to evaluate the health, care requirements, and behavior of each specimen—not just the way the fish’s colors will complement your living room furnishings—before buying. In other words, when choosing fish, you must do your homework and listen to your head, not your heart! Here are the factors you need to consider

Does the Fish Look Healthy?

Look for obvious signs of disease on the fish’s body or any behavior that suggests the fish might be ill. This takes some careful observation, not just a cursory glance, so be sure to watch the specimen for a sufficient amount of time. Some of the warning signs include white spots or velvety patches, torn or incomplete fins, missing scales, cloudy or bulging eyes (unless bulging eyes happen to be normal—as with bubble-eye goldfish), necrotic ulcers or lesions, rapid breathing, shaking or shimmying of the body, scraping against rocks or other objects in the tank, erratic swimming, listlessness, gasping at the surface, or sulking in a corner.

Not only should the specimen you’re considering be free of these symptoms, but so should any other fish displayed along with it. If there is one diseased or dying fish in a tank, you should assume all the other fish in the tank are infected as well, even if they aren’t currently showing any overt symptoms of disease.

How Big Will It Get?

Many a hobbyist has made the unfortunate error of buying an irresistibly cute little fish without first researching its growth potential. Sometimes, that adorable, diminutive specimen grows into a huge, tankbusting terror that is simply too mammoth to maintain in any reasonably sized home aquarium. For instance, that comely little redtail catfish Phractocephalus hemioliopterus may be only 4 inches long when you buy it, but it can ultimately reach a whopping 4 feet or more in length, with an appetite to match. Make sure any fish you buy won’t ultimately outgrow your aquarium, or be prepared to upgrade your tank size to keep pace with the fish’s growth.

In addition to tankbusting potential, it’s important to consider the maximum adult size of a fish relative to the other species that will be sharing its tank. It’s natural for big fish to eat little fish—even herbivorous species will do so opportunistically—so keep that in mind when choosing fish for a community tank. If one fish is small enough to fit in another’s mouth, you should assume it will end up there at some point in the future.

Is the Fish Peaceful or Aggressive?

Fish vary in their aggressiveness toward members of their own species as well as toward members of other species. Some, such as the endearing Corydoras catfish, seem to have a “live-and-let-live” outlook on life, getting along just fine with most other peaceful tankmates, while others seem to be perpetually scrapping for a fight. Then there are those that are relatively peaceful most of the time but take territoriality to the extreme when spawning, or as with the male Siamese fighting fish Betta splendens, when confronted with another male of their own kind. Still others, including some triggerfishes, have a reputation for pulling a “Jekyll and Hyde” routine, lulling their keeper into complacency with their seemingly passive disposition and then suddenly turning murderous toward their tankmates.

The bottom line is that it’s extremely important to familiarize yourself with the general disposition of any specimens you plan to keep before you attempt to put them together. Otherwise—unless you’re really lucky—there will be no end to the squabbling. Or worse, the squabbling will end only after all of your less-aggressive specimens have perished.

Is the Fish Eating?

Refusing to eat can be a sign that a fish is ill, overly stressed, or simply that it doesn’t recognize the item being offered as something edible, because it bears no resemblance to the foods it eats in nature. Therefore it’s a good idea to verify that any fish you’re interested in buying is eating before you purchase it.

Don’t be bashful about asking your dealer to feed the specimen right in front of you. Seeing a fish eat is no guarantee that it won’t present some feeding challenges down the road, but it is reasonable assurance that the specimen is willing to at least sample standard aquarium foods. Some species are extremely finicky or will eat only a specific type of food that cannot realistically be offered in captivity. Certain species of butterflyfish, for example, feed exclusively on coral polyps and will refuse anything else that is offered, no matter how appetizing it might seem from our human point of view.

Is the Fish Captive Bred or Wild Caught?

Whenever you have the option, it’s best to purchase captive-bred fishes over wild-caught specimens. This isn’t such a big issue on the freshwater side of the hobby because the vast majority (though not all) of freshwater fishes sold today are bred in captivity on commercial fish farms. However, just the opposite is true on the marine side, where most specimens are wild caught and only a handful of species are captive bred. The good news is, some of the more popular marine species, including various clownfishes, dottybacks, basslets, gobies, blennies, seahorses, and others are currently being bred in captivity—and the list is steadily growing.

Why does it matter whether you buy captive-bred as opposed to wild-caught specimens? In addition to minimizing collection pressure on wild fish populations and their native habitats, buying a captive-bred fish significantly increases your odds of keeping the specimen alive. Captive-bred fish are generally hardier and adjust more readily to living within the confines of an aquarium than their wild-caught counterparts. Also, they’ve typically learned to accept the standard prepared foods available to hobbyists, such as pellets, flakes, and frozen formulations, which takes a lot of the frustration and guesswork out of enticing new specimens to feed.

Captive-bred fish may cost a bit more than wild-caught specimens, but their superior survival rate and ease of feeding are well worth a few extra dollars and can actually translate into savings, since you don’t have to continually shell out cash to replace deceased specimens.

Making Introductions

When introducing fish to an aquarium, whether it’s a new or established system, there are several additional factors to consider, including water-chemistry requirements, which region of the water column the fishes prefer, territoriality, disease prevention, and proper acclimation.

Can They Share the Same Water Chemistry?

For the most part, any fishes collected from the coral reefs, which are among the most stable environments on earth, will thrive in water of the same chemical makeup—salinity, pH, alkalinity, calcium level, etc. While it can be challenging to provide and maintain the exacting water conditions that reef fishes require, you don’t have to worry whether a royal gramma Gramma loreto collected from the tropical western Atlantic will be able to thrive in the same water conditions as a yellow tang Zebrasoma flavescens collected in Hawai‘ian waters.

The picture is a bit different for freshwater fishes, however, as they have evolved to fill an incredibly diverse variety of ecological niches around the world. While most commonly sold species, having been captive bred, will adapt readily to a wide range of water conditions, some are fairly exacting in their requirements. For instance, cichlids collected from Africa’s Lake Tanganyika require hard, basic water while wild-caught discus Symphysodon spp. collected from South America need water that is soft and acidic. Hence, combining Tanganyikan cichlids with discus would not be your best choice.

Which Level Does the Fish Prefer?

Most fish can be placed into one of three general categories based on the level of the water column they tend to occupy: top-water, mid-water, and bottom dwellers. Oftentimes, you can make an educated guess as to which level a fish prefers based on the position of its mouth. An upturned mouth, which is well suited to snatching food items from the surface of the water, is indicative of a top-water dweller; a forward-facing mouth suggests a mid-water dweller; and a down-turned mouth, which is designed to suck up food from the substrate, is typical of a bottom dweller.

Why does any of this matter? Well, an aquarium is more aesthetically pleasing when the fishes are distributed throughout the entire water column, rather than being all clustered together at the top, bottom, or middle. Furthermore, you’ll have fewer territorial squabbles to contend with if your fishes are spread out among the different niches in the water column, rather than being forced to compete for the same space.

How Do You Prevent Territorial Aggression?

Even fishes that are generally considered compatible will sometimes fight with each other within the confines of an aquarium, depending on the order of their introduction. For instance, if the first fish you introduce to the tank happens to be the most aggressive of all the species you plan to include in the community, it’s very possible that any fishes introduced afterward will be treated as invaders and attacked. On the other hand, if you introduce specimens starting with the most peaceful species, the more easy-going specimens will have a chance to settle in and establish territories before any bullies come along to claim the entire tank for themselves.

Problems with aggression can also arise when a new fish is introduced to an already-established community—with the “new fish on the block” typically being on the receiving end of the hostility. A technique that often helps to minimize aggression in this circumstance is to rearrange the rocks and/or decorations in the tank before introducing the newcomer. This “reshuffling of the deck” disrupts any existing territorial boundaries, putting all of the fish in the tank on a relatively equal playing field, so they have to devote their energies to finding a new territory rather than defending an existing one from interlopers.

How Do You Keep Disease Out of Your Aquarium?

Looking over each specimen at the pet shop for evidence of illness is only half of the disease-prevention protocol for aquarists. Just as humans can be infected with cold or flu germs without showing symptoms right away, fish can be carrying a number of infectious diseases or parasites without exhibiting any obvious external signs—therefore it’s of the utmost importance to quarantine any new fishes for at least four weeks before introducing them to your display tank. If you simply bring your fish home from the store and release them right away into the aquarium without a quarantine period, it’s almost guaranteed that you’ll eventually end up with a major disease outbreak that infects—and possibly wipes out—all of your fish.

Quarantine also provides a safe, protected environment where fish can become gradually acclimated to captive conditions without the stress of having to compete for food and territory with tankmates. Plus, if a specimen does become ill, it’s preferable to administer any medications or other therapies in a quarantine system rather than in the main display tank.

How Do You Set Up a Quarantine System?

A quarantine tank does not have to be large, sophisticated, or expensive. In fact, in many ways, the simpler your quarantine system is, the better. The only items you’ll need to provide are the tank (a 10-gallon tank works fine for many applications, but you may need something larger depending on the size of the specimens you plan to keep), a cover, a heater, a simple air-driven sponge filter, a few places for your fish to hide such as sections of PVC pipe or flower pots (turned on their sides), and a source of water movement and oxygenation, such as a powerhead or airstone. Frequent, large water changes should be performed to keep dissolved pollutants to a minimum while your specimen is in quarantine.

How Do You Acclimate a Fish?

No doubt, the quality of your aquarium’s water will differ at least somewhat from the quality of your dealer’s water—whether in temperature, pH, salinity in the case of marine fish, or some other parameter—and sudden exposure to different water conditions is, at best, stressful to fish and, at worst, deadly to fish. So any time you introduce a fish to either your quarantine tank or, following quarantine, to your display tank, you must first acclimate it to the water in its new environment.

Acclimation should be accomplished slowly, drip by drip. First, gently pour the fish and the water from the plastic bag into a clean plastic container. Then take a length of flexible airline tubing and tie a few knots in it. Place one end of the tubing in your quarantine or display tank, and place the other end in your acclimation container. To start water flowing through the tubing, you can suck on the end of it, but a better (and more hygienic) approach is to place the opposite end of the tube in front of a powerhead or the return flow of your filter system. Once the water is flowing, tighten or loosen the knots in the tubing to adjust the flow rate to a steady drip.

Continue dripping until the volume of water in your acclimation container doubles, then pour out half of the water and resume dripping. Repeat this process until the water parameters in your acclimation container are identical to those of your quarantine or display tank. You’re now ready to release the fish into its new home.

You’re on Your Way!

Following these few basic recommendations when selecting and stocking fish will help you sidestep many of the common errors that lead to failure with fishkeeping, which cause many people to abandon the aquarium hobby before giving it a fair chance. Doing it the right way the first time around requires patience and diligence, but rest assured, your efforts will bear fruit in the form of a beautiful, healthy, and vibrant community of fishes.

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