Stocking Your Freshwater Aquarium

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Glass Catfish make good additions to peaceful community tanks.

Creating a Balanced Fish Community

By Scott W. Michael

As if it were yesterday, I can clearly remember walking into a tropical fish store for the first time. Walls of glowing tanks, packed with piscine gems. The place had the warmth, humidity and distinctive aromas of a well-kept aquarium shop, more than slightly suggestive of the exotic tropics. It was sensory overload for a young mind.

Within the tanks was a multitude of shapes, colors and behaviors. Some fishes were dashing about the upper layers of the aquariums. Some were peeking out from behind rocks or weaving in and out of thickets of plants. Others rested on the bottom or adhered to the glass walls of their aquariums.

Within minutes, I had found dozens of fishes that I wanted to take home to keep in my own 10-gallon ecosystem. Fortunately, the store owner was a conscientious fish enthusiast and he gently guided me toward a few hardy fish that would help me establish my new aquarium, fish that would survive as I learned the basics of fishkeeping. He also introduced some concepts that I had never considered: selecting fishes that occupy different “layers” from top to bottom, those that gravitate to different microhabitats within the tank and choosing fishes that perform important tasks in the aquarium, like algae-eating and scavenging uneaten food from the bottom.

A World of Choices

There are literally hundreds of fish species commonly available to the tropical fishkeeper. This amazing selection is often overwhelming to the neophyte. The prudent aquarist will not buy every fish that catches his or her eye, but will stop and ask: Which species are right for my aquarium community?

Not all the fishes dashing and darting about the tanks before you are suitable for the community aquarium. They may have different preferences when it comes to water chemistry parameters. Some are very placid and may fall prey to the more pugnacious or predatory species that also regularly inhabit the tanks of the average pet store. Of course, the members of the latter guild will need to be housed on their own or with other rough and tumble characters.

Redtail Catfish.

It is essential to know that many freshwater species offered for sale also attain some amazing sizes. Isn’t fish growth suppressed if they are housed in a smaller tank? Just ask anyone who has purchased a juvenile Phractocephalus hemioliopterus (a.k.a. the Redtail Catfish) if the growth of these fish is constrained by the size of the tank they are kept in? (Is a St. Bernard puppy brought home to a tiny apartment going to stay small and cute?) The Redtail Catfish can outgrow a smaller tank in no time, and has the capacity to turn into a 4-foot beast.

Most species available to hobbyists do not reach such gargantuan proportions, but it is still important to consider the maximum size of a fish species before you plunk it into your 20-gallon tank.

Piranhas on the Loose

Anyone who has worked at a tropical fish shop has seen the results of aquarists making bad choices. In the Midwest, there are numerous stories of startled fisherman pulling large, piranha-like fish out of local lakes. More often than not they are Pacu—big characins belonging to several different genera — not piranhas.

These piscine trespassers are usually freed in these lochs because they have outgrown their aquarium homes. Not only does this lead to eventual death for the overgrown tropical (the cold winter will kill it), when exotic introductions are carried out in warmer climes it can lead to ecological disaster.

A non-resident fish can have a very deleterious impact on native species, disrupting ancient biological balances and even threatening the very survival of indigenous fishes. Florida and other southern states have many horror stories of aquarium fishes released into local ponds and streams. Major initiatives are underway to discourage hobbyists from dumping their problem fish into wild waterways.

Red-bellied Piranha.

Rash purchases of certain species can also wreak havoc in the home aquarium. I have had friends ask me to visit their tanks to see why they were not having any success, only to find, for example, what was analogous to a 24-hour “cage match” between a colorful Zebra Cichlid (a well known antagonist) and various South American river-dwelling fishes that were cowering in the corners of the tank trying to avoid the domineering bully. Once the territorial African cichlid was removed and returned to the local fish store, things settled down and the mortality rate of the fish in the tank plummeted. It is truly amazing how much unhappiness, loss of fish lives and expense is incurred by hobbyists making naive or reckless choices when they bring home new fishes.

One of the best ways to avoid these problems is to know something about the charges you are going to keep. Knowing some of the biology and natural history of each species not only makes watching them more interesting (you’ll understand the unusual behaviors they may exhibit), it will also help you place fish together that will get along and share similar environmental requirements. The aim of 101 Best Tropical Fishes is to help you make wise choices so you will greatly increase your chances of success rather than having to learn the hard way and at the expense of unfortunate fishes.

The Layering Effect

When selecting fish that will make up your aquarium community, you will want to choose species that occupy different microhabitats in the aquarium. If you have ever engaged in landscape design, you know all about microhabitats and creating the layered look. You have your ground cover and shorter plants along the bed borders, your medium-height varieties in the middle of the bed, and your taller forms near the back of the landscape. It looks more interesting and prevents your plants from crowding each other out. We need to utilize a similar methodology when selecting fish for our tanks.

While fish are not sedentary like plants, there are many that will occupy various “layers” of the aquarium’s water column. There are benthic species (bottom-dwelling) that hang near or live on the gravel bed. There are species that tend to hover or swim about in the mid-levels of the aquarium and there are species that tend to stay near the water surface. There are also some species that occupy all three layers of the tank. To make your aquarium more interesting, and to provide more living space for your fishes, you will want species that hang-out in all three layers of the water column. Obviously, these differences in microhabitat use will be more pronounced in a deeper tank.

Ditherfish & Schoolers

In aquarium circles, ditherers are important and well liked. Dither fishes are active species that are bold and spend most of their time darting about in the open. Their presence serves to have a calming affect on the rest of the fish community. More reclusive or nervous fish species will cue in on the behavior of their neighbors to determine if it is safe to come out of hiding. If these fish are not kept with dither species, they are likely to spend more time under cover and less time where the aquarist can observe and enjoy them. Some good dither species include danios and rainbowfishes.

There are also some species that naturally school and will do poorly if kept on their own. These species require the presence of conspecifics to feel secure. Most of these fishes school because there is safety in numbers. Therefore, when they are on their own or in groups that are too small (just two or three when they are accustomed to many more), they sense that they are vulnerable and as a result they are edgy, they feed poorly and are often less colorful. There are tetras, danios, and rainbowfishes that fall into this group. A single Neon Tetra is a lonely and, most likely, very uncomfortable fish.

Dealing with Aggression

Aggressive fishes can cause real problems in the confines of the aquarium. Aggression causes stress that can make your fishes more susceptible to disease, even if it doesn’t kill a fish outright. An accosted fish may behave abnormally, hanging around in the upper corners of the tank or hiding incessantly. It may eat less and end up starving. It may also exhibit subdued coloration. It is up to us as fishkeepers to populate our tropical fish communities carefully in order to avoid aggression issues. However, there are times when the best-laid plans can be fouled up by a fish that turns into a bad actor.

There are several techniques to help limit the likelihood of aggressive encounters between your fish charges. First of all, and this bears repeating, do not put known belligerent species with those that are timid or peaceful by nature. Always consider a fish’s propensity for aggression before turning it loose in your community tank.

Of course, many aquarists are attracted to big, rambunctious or even highly predatory species. If you do have a hankering to house a hellion, be sure its tankmates are like-minded. (The color tags on the edges of each species page will help you delineate between fish of various dispositions.)

To make things interesting, however, there are numerous species that are not always pugnacious but can become combative or territorial in the confines of a small aquarium. There are also species that fall on or near the line between being peace-loving and moderately aggressive. With these there are a variety of things that might contribute to how they behave in the confines of a home aquarium. These factors include the size of the tank, the density of fish in the community, the order of their introduction to the aquarium, the abundance of hiding places and the physical/behavioral characteristics of their tankmates.

Size Matters

Aquarium size does matter when it comes to aggression. You will have fewer problems with antagonism in a larger tank. Even if you are keeping known bullies, in a large tank there is often enough room for other fishes to avoid them. If the tank is deep, it is likely that substrate-bound species, which tend to be more bellicose, may rarely encounter those fish that live near the water’s surface.

With some fishes, you will have better luck if you stock them at higher densities. This, for example, is true with African cichlids. These fish typically form dominance hierarchies in the aquarium. If you have a few in the tank, the individuals at the bottom of the pecking order with be bullied, sometimes to death, by those higher up on the social ladder. However, if you place enough individuals in the tank, a defined hierarchy is more difficult to maintain. Instead of one or two individuals being constantly pestered, aggression is often more evenly distributed throughout the population, with no one fish getting bullied to death.

When putting together your fish wish list, be sure to rank the species according to their propensity to be aggressive, with the most passive species at the top and the most belligerent species at the bottom. This is how you should introduce them to the aquarium—least aggressive first, most aggressive last. This works because prior residence will often impact who is the boss.

Fuelleborni African Cichlid

If a potential troublemaker is added to the tank first, it is more likely to lay siege to a territory and defend it fiercely against any newcomers. On the other hand, if you start populating the tank with the least territorial fish, there is less likelihood that a moderately aggressive species will later challenge fish that already consider the tank their home. Of course, the always-aggressive species should never be housed with the peaceful types, unless the aquarium is very large. For example, you could have some of the moderately aggressive cichlids in a tank with larger tetras, if the tank is, say, in the 125-gallon (474 L) range and has plenty of places for fishes wanting to seek shelter.

The Habitat Factor

This brings us to the third factor impacting behavior: aquascaping or tank interior decor. If an aquarium lacks hiding places, then there is more likelihood that the few places available will be defended vigorously by site-attached fish species such as cichlids. However, if there are plenty of hiding places to go around, these squabbles will be less frequent. Also, if a tank is packed with live plants, some of the more passive mid- and upper-water swimmers can seek shelter among the foliage if they feel threatened. Not only does this help prevent their being picked on, it also provides a sense of security that reduces stress levels and allows fish to look their best.

Finally, those fishes that defend a territory tend not to be indiscriminate about who they chase or nip. In the wild, this could be costly to the attacking fish as aggression takes them away from more important activities, such as eating and reproduction, and can make them more prone to being picked off by a lurking predator. Therefore, they tend to go after those species that compete with them for resources, like food, shelter and mates.

Profiling Your Fishes

For the reasons stated, a territorial fish is going to be most aggressive toward conspecifics (they have the most resource overlap). Next on the list are usually those species that have similar food habits. For example, an algae-grazing cichlid is most likely to assault another fish that shares its dietary proclivities. But how do they know which fish are food competitors? Typically, fish that feed on the same foods are similar in form and behavior. Therefore, a territorial fish will usually attack another fish that looks like it (form and/or color) and one that behaves like it. There are some other ways to reduce the likelihood of aggression when you are adding a new fish to the tank. Some of these techniques can be especially useful when placing moderately aggressive or highly aggressive fishes together in the same tank.

Finally, I like to use the analogy of buying a new puppy. The happiest dog owners are the ones who spent some time researching the differences between breeds before making their final choice. Tropical fishkeepers who make the effort to ask questions about fishes that catch their eye and who make the effort to look up the profile of a new species are invariably happier with their aquariums.

It is also worth knowing that, with reasonable care, certain tropical fishes can live more than 10 years, and 20 years is not unheard of. Choose the right fish and it has the potential to become a pet for years to come. Using this and other guidebooks, tapping the experiences of other aquarists and informed store staff members can go a long way to helping you evade the predictable headaches and heartbreaks that result when we put the wrong species together in the microcosmic world of a home aquarium.

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Excerpted from: 101 Best Tropical Fishes
Chapter by: Scott W. Michael