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Issue: August 2007

The Subtropical Aquarium: A Cooler Kind of Fishkeeping

Author: Neale Monks


Photographer: Horst Linke
There are some interesting and hardy species of aquarium fish that prefer conditions somewhere between the temperate and tropical temperatures of most aquaria.

We’ve become so used to the tropical vs. coldwater aquarium dichotomy that it’s easy to forget that the natural world is really much more subtle. In between the tropical and the temperate zones are places that blend characteristics of the two, and they are known as the subtropics.

Subtropical climates exist in places like Florida and Arizona, southern China, and the northern highlands of India, where it is usually very warm in summer but rather cooler in winter. Such climates are much warmer than those in the temperate zones, with temperatures low enough for snow or ice rarely occurring, but there is still much more variation between summer and winter than there is in the tropics. Animals and plants adapted to subtropical climates are therefore much more adaptable than tropical species, even if they lack the ability to tolerate freezing temperatures for long. In short, subtropical animals and plants like mild temperatures rather than hot ones, but they don’t mind fluctuations up or down provided they’re not too extreme or protracted.

Unsurprisingly, perhaps, subtropical aquarium fish are among the hardiest species and include several that were kept during the very earliest development of the hobby. Perhaps best known is the paradise fish Macropodus opercularis, which naturally occurs in China and Korea. First imported into Europe during the mid Nineteenth Century, its ability to thrive in unheated aquaria as well as its attractive markings quickly made it a staple in the hobby. Although later overshadowed by the more showy gouramis and bettas, it is still regularly traded and easily obtained, and it makes a good candidate for a community tank containing other hardy and robust species.

Why Keep a Subtropical Tank?

The prime reason to set up a subtropical aquarium is to keep some very interesting species that are otherwise not long-lived in tropical or coldwater tanks; these include freshwater blennies, hillstream loaches, and dragon gobies. There are also plenty of nice schooling fish, like White Cloud Mountain minnows and ruby barbs, that provide action and color, and for the most part all these fish will get along well together.

A subtropical tank is also relatively inexpensive to run, because even in winter the heater will be doing nothing more than keeping the chill off the tank, and during summer the aquarium might not even need to be heated at all. In centrally heated homes the aquarium can be allowed to warm and cool with the house, provided the temperature does not fall much below 18°C (64°F).

A third and perhaps most compelling reason to keep a subtropical tank is that they can accommodate many popular coldwater species as well. Goldfish, especially fancy varieties, will thrive in a large subtropical tank, as will weather loaches and most sunfish. Thus a subtropical aquarium can be an ideal choice for the aquarist who already has a goldfish that has become a much-loved pet, but would like to add some pretty schooling fish for a bit of movement and variety. Rosy barbs in particular are the right size and temperament to go well with goldfish, and their metallic colors complement those of goldfish very nicely.

Creating Subtropical Conditions

The defining characteristic of the subtropical aquarium is temperature. Excessively low temperatures (as in a completely unheated aquarium, or outdoors in a pond during the winter) will stress these fish and make them prone to parasites and other diseases. Unlike true coldwater fish, such as goldfish and sunfish, these fish have little resistance to freezing conditions and cannot survive under ice. So while many will do well outdoors during the summer months, they will need to come in once it starts to turn colder in autumn. A minimum of 18°C (64°F) is about right for most of the subtropical species commonly traded.

Keeping the aquarium too warm is equally undesirable, with a temperature of 22°C (72°F) representing a comfortable upper limit for most of these fish. This obviously contrasts with truly tropical species, for which such a temperature would be rather low, and the 24°C (75°F) temperature that most people keep their tropical aquaria at is unhealthy for many of these species. If kept too warm, these fish tend to have short lives, partly because their metabolism is speeded up, but also because the warm water contains less oxygen that they are used to.

Another contrast with tropical species is that many subtropical species do best if there is some variation between summer and winter temperatures. In fact, allowing the tank to cool down in winter but warm up again in spring does seem to trigger spawning in many of these species, as with rosy barbs and white cloud mountain minnows. A standard aquarium heater set to a low setting should keep an aquarium warm enough for these fish during the winter time, and in summer you can turn the thermostat on the heater up to a slightly higher setting. Use a thermometer to keep track of temperature, and if the tank gets too warm in summer, switch the heater off and provide some extra aeration to keep the oxygen levels up.

Planting a subtropical tank isn’t difficult because many aquarium plants will do fine at the lower temperatures that are required. Suitable plants include most species of Cabomba, Eleocharis, Ludwigia, Sagittaria, and Vallisneria, as well as Bacopa caroliniana, Cardamine lyrata, Ceratophyllum submersum, Echinodorus cordifolius, Echinodorus tenellus, Egeria densa, Hydrocotyle vulgaris, Hygrophila polysperma, and Nuphar japonicum. Although only a borderline aquarium plant under most circumstances, the Japanese rush Acorus gramineus does seem to do better permanently submersed in subtropical tanks compared with tropical ones. You could also add Aponogeton crispus, a wonderful plant that frequently flowers in aquaria but is often treated badly by aquarists. Because it needs a “resting period” during the winter, the aquarist with a subtropical tank can easily remove the plant in late autumn, cut away the leaves (which will be looking a bit sad by now anyway), and then store the tuber in a jar of water somewhere cool and dark. Once spring returns, the tubers can be returned to the tank, and the seemingly dead tuber will set forth new leaves.

Species Roster

The following species include most of the commonly traded subtropical species, plus one or two others that turn up occasionally and are well worth seeking out. I’ve stuck to small, sociable species here to keep things simple, but there are also many large and predatory species such as bass, gar pike, and channel cats that do well at subtropical temperatures even though they are more often kept as coldwater species. Of course the problem with these “tankbusters” is that they need large tanks and will view smaller tankmates as nothing more than live food, so they don’t make ideal community fish.

In contrast, all the fishes listed below are more or less peaceful, non-predatory species, although it would be unwise to mix species that are very different in size. Just as adult angelfish will think nothing of eating your school of neon tetras, a black-banded sunfish might well snap up a dwarf mosquitofish if given the opportunity.

Schooling Fish
Rosy barb Puntius conchonius

A superb fish for the home aquarium, both sexes have a lovely metallic sheen, while the males top this by developing a warm pinkish tint to the flanks and belly. Potentially able to reach as much as 15 cm (6 inches) in length, most aquarium specimens stay substantially smaller, but even so, this is a large and lively fish that needs space and good filtration. Although quite big by aquarium fish standards, it isn’t at all aggressive or territorial and it does not nip fins. About its only bad habit is a tendency it shares with other large barbs to root about the substrate, and in doing so it may disturb recently planted cuttings; but it is otherwise a good fish for the planted aquarium. If very hungry it might nibble at soft leaves or shoots, but otherwise it prefers a mixed diet of flakes, frozen foods, and algae wafers. The rosy barb is large enough to do well with goldfish and paradise fish, provided the tank is big enough and the filtration is up to the job.

The rosy barb was one of the first species to be regularly maintained as an aquarium fish in Europe and North America, and a few artificial strains have been created. The long-fin rosy barb is easily the most common, and this can be a most attractive fish provided it is not kept with nippy species like mosquitofish. Breeding isn’t difficult and is accomplished in a similar way to other barbs, with the parents scattering the eggs over fine-leaved plants.

Golden Dwarf Barb Puntius gelius

One of my very favorite fish, these are lovely barbs with a semi-transparent body and yellow and black markings on the sides and back. Although not the most active barbs, they are sociable; a group of six or more makes an excellent addition to any subtropical aquarium with other comparably sized species. Unlike some better-known barbs, these fish aren’t fin nippers, and they can be trusted to get along with even very docile fish like gobies and minnows.

Golden dwarf barbs don’t place very many demands on the aquarist, readily eating dry foods as well a frozen and live ones, and will even nibble at a little green algae if there is some. Ideally the water should be slightly acidic, but they seem to do well in neutral to slightly alkaline water conditions as well. Like many other subtropical fish, these Indian barbs are a bit sensitive to low oxygen levels, and this is one reason that they are often considered delicate by aquarists who keep them in ordinary community tanks. They also appreciate plants, particularly floating species, and they like to hang about in the shade rather more than in brightly lit areas. Nonetheless, when settled in they are lovely fish well worth keeping.

Green Barb Puntius semifasciolatus

The green barb isn’t all that green, most specimens being some shade of yellow, sometimes coppery, but more often rather golden. Either way, these are handsome, active fish that are quite a bit hardier and easy to look after than the dwarf golden barb. They are very lively, and thus need a good deal of open swimming space. They are also rather large barbs, commonly 8 to 10 cm (3½ to 4 inches) in length. Since they are schooling fish they are best suited to fairly large aquaria.

Originally from China, these barbs appreciate cooler temperatures in winter and warmer ones in summer, so allowing the temperature of the aquarium to vary with the seasons is a good way to condition them for breeding. As with most barbs, they will eat pretty much anything, including flake and dry foods, but also frozen bloodworms, algae flakes, and small live foods such as Artemia.

White Cloud Mountain Minnow Tanichthys albonubes

The White Cloud Mountain minnow used to be called “the poor man’s neon tetra,” and it is certainly true that this Chinese fish has the same lively personality and bright colors as that characin. It isn’t at all difficult to keep, and being easily bred on a commercial scale, it is among the least expensive tropical fish around. Most are kept in tropical tanks, where they do fairly well but tend to be rather short lived. In a subtropical tank you will see these fish at their best—the higher oxygen levels really bring out their happy dispositions, especially in a tank that catches a bit of sunshine in the morning. As with most minnows these fish are very sociable, and the bigger the school the more impressive they will become. Since they are small and so easy to obtain, there’s really no reason not to keep a group of 20 or more.

If kept cool during the winter these fish are easy to breed. An aquarium with a summer temperature no higher than 22°C (72°F) and thickly planted with feathery plants, such as Java moss, is ideal. The fish will scatter their eggs and then ignore them. After a few days the eggs will hatch, and the fry can be offered newly hatched brine shrimp and commercial egg-layer fry food.

Mosquitofish Gambusia holbrooki

The mosquitofish is something like a sturdy, rather aggressive version of the guppy. While it lacks the bright colors of the artificial strains of guppy most aquarists are familiar with, in shape and size in it is quite similar, and like the guppy it is an adaptable fish that doesn’t demand much in terms of water quality or aquarium decor. On the downside, though, these fish are aggressive for their size and will bully smaller species like dwarf mosquitofish and golden dwarf barbs, especially at feeding time. They are also prone to becoming fin nippers, so they cannot be mixed with species with long trailing fins, such as paradise fish.

Mosquitofish were originally found in the southern United States and Mexico, but they have been very widely transported around the world as a biological control for malaria. As their name suggests, these fish feed primarily on insect larvae, especially mosquito larvae, and since mosquitoes are the animals that infect humans with the malaria parasite, the theory is that filling standing water pools and ditches with mosquitofish helps control the spread of malaria.

Dwarf Mosquitofish Heterandria formosa

The dwarf mosquitofish is one of the smallest fish kept in the hobby, but provided it is only kept with peaceful species such as golden dwarf barbs and dragon gobies that won’t view it as food, it isn’t difficult to look after. Unlike the common mosquitofish it isn’t at all aggressive, but sadly this fish isn’t all that commonly traded and remains relatively expensive. Aquarists in the United States do of course have the option of collecting their own specimens (this fish is most common from North Carolina south to Florida).

Maintenance is simple; all this fish requires is a densely planted aquarium and small live and frozen foods (brine shrimps, mosquito larvae, and bloodworms forming an ideal staple). Once settled, the dwarf mosquitofish will also take flake foods and perhaps some green algae as well. Males are smaller than the females and are equipped with a gonopodium, but they are otherwise similar. Females will produce broods of young every few weeks, and the fry will take finely divided flake and Artemia nauplii as well as commercial livebearer fry foods. These fish are totally peaceful among themselves, and they should be kept in groups of six or more, preferably with more females than males.

Variatus Platy Xiphophorus variatus

Although the variatus platy has been popular as a tropical aquarium fish for years, it actually does better at subtropical temperatures. They are hardy, colorful, and completely peaceful, and make an ideal addition to almost any subtropical community tank. Like the common platy, these fish need a mixed diet of small insects and insect larvae along with plenty of greens. Algae is an especially important part of their diet and can be provided either by allowing some to grow in the aquarium, or by feeding them a vegetarian flake food such as those produced for mollies and cichlids.

Breeding is easy provided the fish are fed well. As with most livebearers, these fish will probably breed without much intervention from their keeper; if not, raising the temperature to around 25°C (77°F) appears to be a most reliable trigger. If you do this in the summer it won’t upset any other subtropical species in the aquarium with them, and in fact may trigger the breeding behaviors of others as well.

Midwater Specimen Fish
Spanish Killifish Aphanius iberus

Typical of the European and North African killifish that are occasionally imported, this fish is undemanding and easy to look after. Like most other killifish, it feeds primarily on small insects and insect larvae but will also take small crustaceans, such as Artemia, and clean tubifex worms. The Spanish killifish is peaceful, and while it doesn’t actively school, it does do best when kept in small groups. A densely planted aquarium is important; in an empty, brightly lit tank these fish are skittish and nervous.

Although breeding is carried out in much the same way as with other killifish—the parents scattering the eggs among the leaves of plants—a cool winter temperature followed by a warmer summer is absolutely essential. In this regard it is a lot like many of the other subtropical fish that spawn only in the summer when temperatures are high and the food supply is most abundant.

Paradise Fish Macropodus opercularis

The paradise fish, which is closely related to the bettas and gouramis, is a robust labyrinth fish that is sometimes aggressive but generally adaptable. Adult males are especially intolerant of one another and will attack each other on sight. On the other hand, a single male kept with two or three females in a well-planted spacious tank isn’t any bother at all, and kept like this they are also good community residents. Good companions would include goldfish, rosy barbs, and mosquitofish, but because paradise fish have long fins and are quite slow moving, it is important to avoid any species that are fin nippers.

Paradise fish are relatively easy to breed, the trigger for spawning appears to be raising the temperature from the normal subtropical level to a more tropical 24°C (75°F). Males have brighter colors and more elongate tips to the dorsal and anal fins compared to the females. Like many other labyrinth fish, the male builds a bubblenest, and after spawning he will drive the female away and guard the eggs alone. The fry are small and will need infusoria or commercial egg-layer fry foods to begin with.

Black-Banded Sunfish Enneacanthus chaetodon
Pretty but rather delicate North American sunfish, these need to be kept in neutral or slightly acidic, densely planted aquaria to do well. Once referred to as “the poor man’s angelfish,” they are now much more expensive and far harder to find than the now-ubiquitous angelfish. Black-banded sunfish are notoriously intolerant of poor filtration and low oxygen levels, and on no account should they be kept in immature aquaria. But in the right aquarium, these are very striking fish with nice personalities.

Black-banded sunfish are not especially territorial, and several specimens can be kept in a large aquarium. Breeding is possible, with the male guarding the eggs and fry until they are free swimming. When not breeding, any other species kept with them are ignored, though large specimens could potentially view very small fish as food. Otherwise these fish readily accept small insects and insect larvae, Artemia, and other similar foods.

Bottom Feeders
Hong Kong Pleco Gastromyzon punctulatus

Hong Kong plecos are neither catfish, nor from Hong Kong. They are in fact from South East Asia, where they inhabit shallow mountain streams, where they use their sucker-like pectoral and pelvic fins to stick to rocks and pebbles. Hong Kong plecos are members of a family of fish known as Balitoridae, or hillstream loaches, and are quite closely related to the carps and minnows. Besides the Hong Kong pleco discussed here, a number of other species of hillstream loaches are irregularly traded and all can be kept in much the same way.

Though commonly traded, Hong Kong plecos are rarely kept properly, and this has resulted in a reputation for being delicate that isn’t really justified. The average community tank is simply too warm for these fish, and between that and the low oxygen levels, they tend not to live for very long. But when kept at subtropical temperatures, and with extra aeration in the summer to keep the oxygen concentration high, these fish do well. They are not picky about pH or hardness, but good filtration is essential. While Hong Kong plecos are sometimes kept in coldwater aquaria they actually do best in heated aquaria kept at a temperature of around 20°C (68°F).

While sold as “algae eaters,” they are really more omnivorous than that and will need a mixed diet including green algae, catfish pellets, bloodworms, and even small pieces of shrimp. Tubifex worms, if clean, are also a good food for these fish. They are not often spawned, but it does happen occasionally, so it is well worth the ambitious aquarist keeping a group of these fish just in case. Although Hong Kong plecos are vaguely territorial, shoving one another off a particularly attractive patch of algae, they are otherwise completely harmless, peaceful fish.

Freshwater Blenny Salaria fluviatilis

Sadly rarely seen, this very interesting European and North African species does badly in tropical aquaria but will settle into a subtropical tank quickly. The main issue with this species (beyond getting hold of it in the first place) is that it is notably intolerant of low oxygen levels and poor filtration. A powerful external filter is probably essential, and aeration will need to be supplemented either by adding a venturi or spray bar to the filter, or by using an air pump.

Freshwater blennies are fairly large when fully grown, reaching around 15 cm (6 inches) in length, and they are very territorial. Each fish will need its own cave or burrow, and ideally only a single male should be kept in any one tank. Otherwise these are not difficult fish, and they will adapt to a mixed diet of live and frozen foods, and once acclimated to the aquarium many specimens will also take flake and pellets as well. They breed in much the same manner as gobies, with the males taking care of the eggs.

Dragon Goby Rhinogobius duospilus

Often sold under an obsolete Latin name, Rhinogobius wui, these Chinese fish are truly wonderful specimens that combine liveliness and interesting behavior in a pretty goby-sized package. Unlike many other gobies (such as bumblebee and knight gobies) these fish don’t need salt in the aquarium and are in fact quite tolerant of a range of water conditions, although neutral to slightly alkaline water suit them best. Dragon gobies are completely peaceful toward other fish, but the males are territorial and will defend their turf vigorously, though not to the point of harming one another.

Dragon gobies pose no particular problems for the aquarist. They don’t eat flake or pellet foods but readily accept most small live foods as well as substitutes like frozen bloodworms, Artemia, and lobster eggs. They are very active and greedy, so they won’t miss out at feeding time, as is too often the case with gobies. Breeding presents only a modest challenge, the fish spawning readily, with the male guarding the eggs until they hatch. Once the fry are mobile and feeding, they will accept newly hatched Artemia and other small, live foods.

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