Many fish that exhibit marked differences in colouration are sold as pairs to ensure the sale of the somewhat more plain females but without females to impress, males often fail to display their bright colours. Species such as gouramies fall into this category and are best kept as multiple pairs in large well-planted aquaria. Amongst the few species that do need to be kept as pairs are some of the monogamous cichlids but even these prefer to select their own mates and are best kept initially in groups when young. 

By contrast, most marine fishes are territorial and aggressive and should be kept singly in the case of average-sized aquaria. Some species can be kept in pairs or small groups but these are the exception and should ideally be seen together before purchase and introduced simultaneously for best results.

The overwhelming majority of tropical fishes seen in shops are bred in tanks or ponds and may represent forms that are unknown in the wild. Free from the pressures of predation and competition, albino and long-finned forms of many species have been bred and would not survive in a wild situation. Due to environmental damage, a number of species familiar to aquarium keepers are threatened or even extinct in the wild such as the Redtailed Black Shark (Epalzeorhynchus bicolor) or the Cherry Barb (Puntius titteya) and for these, popularity as aquarium fish ensures their survival.

 

A much smaller proportion of fishes in the trade consist of those that are collected from the wild. In the case of freshwater species, many of these are harvested from areas where massive seasonal fluctuations in water levels mean that a surplus of individuals are destined to die in the dry season and represent a very sustainable means of income for local people. As such, these small fishes remain a valuable commodity to be protected and with them their habitat - ensuring the preservation of biodiversity and discouraging destructive land use. As long as the species are those that will thrive under captive conditions, such sustainable use of a renewable resource is to be encouraged and in these enlightened times valuable work in nature reserves can be financed by the judicious collection of wild-caught fish.

 

Unlike their captive-bred counterparts, wild fish are used to optimal water conditions and will often prove problematic if your aquarium water is high in pollutants such as nitrate or phosphate. Make sure that these levels are acceptably low before introducing any new fishes to your aquarium.

Like trees in a forest, corals are in competition for space and light in a reef environment. Because of this, they have evolved ways to defend themselves against their neighbours and these involve stinging with so called 'sweeper tentacles' or indulging in chemical warfare.

Many species that find themselves thrown together in captivity do not occur together in the wild and it is often observed that some corals are prone to damage from neighbouring animals. Soft corals tend to use toxins to fight for space and it is often wise not to mix them with more demanding SPS (small polyped scleractinian) hard corals such as Acropora, a situation that is easily avoided by noting the differences in light levels and current that these animals prefer. LPS (large polped scleractinians) hard corals such as Galaxea are more likely to nettle their neighbours using long tentacles and need to be widely spaced to avoid injury.

Coral reefs are an island of life in otherwise low nutrient seas. Most of the resources are tied up in the tissues of plants and animals and oportunities are limited. Predators are all too eager to consume any animals that lack shelter and any fishes sleeping in the open at night will not often live to see the next morning. If you possess a good hiding place and a food source, there is always someone wanting to take it from you!

Typically, less territorial species feed on abundant food sources such as plankton or open water shoaling fishes. They may be shoaling so they can overwhelm individuals that would successfully guard their food source from smaller numbers of fishes and the large shoals of Tangs such as Convict Surgeons (Acanthurus triostegus) are an effective way of feeding on the algal gardens of other species.

Sex is also an issue close to the hearts of most marine fish and the fact that many species compete for the role of dominant male or female serves to make them even more aggressive.

Not all corals live in sun-drenched shallows, or grow slowly enough to gain all their nutrient needs from their symbiotic algae.

Depending on polyp size, corals catch various planktonic organisms from the surrounding water and will often display a distinctly different appearance at night or when tides and currents bring feeding opportunities. Feeding tentacles equipped with stinging cells enable them to overpower small animals and some of the most remarkable behaviours are displayed by soft corals such as the Elephant Ear Mushroom (Amplexidiscus fenestrafer) which mimics anemones and devours small fish such as clowns which shelter in them.

If you live permanently attached to a site under the tropical sun, it makes sense to be solar powered. Although this is common in plants, animals don't possess the adaptations to photosynthesize and this is where the symbiotic relationship between algae and corals gives both organisms a great advantage.

The algae in question - known as 'zooxanthellae' live within the cells of corals, anemones and giant clams and provide sugars to their host, in exchange for shelter and nutrients in the form of carbon dioxide and nitrogen based waste.

In the complex environment of a coral reef, there are huge numbers of species and many of them are in competition. One way to beat the opposition is to specialise in one or two specific food sources or ways to harvest a meal.

An example is the long nose of the Copperband Butterflyfish (Chelmon rostratus) which enables the fish to feed in small crevices inaccessible to shorter nosed species that would otherwise eat the same items. The astonishing feeding technique of the Dragon or Rockmover Wrasse (Novaculichthys taeniourus) is another way of finding food and a pair of adult fishes will alternate between lifting rocks and grabbing small animals sheltering beneath.

A common strategy is to find a food source that other species don't use and avoid competition entirely. Many Butterflyfishes feed on coral polyps of different species, enabling them to live in the same areas without competing with each other. Another strategy is to guard your food source and a number of Tangs, Damselfishes and Blennies are 'Algal Gardeners' and exclude all competitors from their territory where they farm the algae that feeds them - often removing species that they don't eat.

Unlike many popular freshwater fishes that naturally live from one wet season to the next, reef fish are built to last. In the wild, the highest mortality rates occur when young fishes settle out of the plankton and have to find a territory of their own - failure results in either predation or starvation. This is why it is always preferable to select small individuals when choosing your marine fish, as this 'post-planktonic recruitment' represents a surplus of youngsters that would naturally fall prey to selection pressures in the wild, rather than removing key breeding adults vital to a healthy population.

Once they have a territory and food source, predation is the greatest issue facing the average reef fish and often the combative nature of fishes such as Damsels means that they can recover from physical traumas that would otherwise prove fatal to less robust freshwater species. There are few marine equivalents of the 'live fast - die young' Livebearers, Tetras and Killifishes seen in home aquaria and so teenage aquarium fish are commonly seen in well maintained reef tanks.

By preventing premature deaths due to water quality, disease or aggression, you can stand a good chance of seeing your reef fish grow to a ripe old age of around ten years for all but the smallest reef fish.

At this point it is timely to add that a number of specialised feeders have no place in the home aquarium and often fail to thrive in aquaria. Species such as Sweetlips, Coral-eating Butterflyfishes and Redfaced Batfish will normally fail to thrive and suffer greatly reduced life expectancies in captivity. This is why we do not agree with their importation and avoid stocking them in our stores.

Black beard algae is actually a red algae and thrives in high-phosphate environments. What this means is that if you have it in your aquarium, it's either a sign that more waterchanges are needed - perhaps boosted by using a gravel cleaning syphon, or that your tapwater contains high levels of phosphate. As ever, a water test can save a great deal of guesswork and help you tailor a maintenance schedule that suits your aquarium.

Areas where old Victorian plumbing is still in use tend to be the worst for phosphate contamination, as water authorities add this to the water to prevent any corrosion of potentially harmful lead piping. To put this into perspective, black algae may be a nuisance but is preferable to a dose of lead poisoning! If your tapwater tests positive for phosphate, it may be better to switch to RO water to avoid adding more algae-food each time you carry out a waterchange.

With inputs of phosphate minimised and solid waste removed from your aquarium, adding a phosphate removing resin to your filter is a good way to starve out the algae. Old fluorescent tubes can also be a problem and with time, their output can shift to favour the growth of algaes rather than more demanding aquarium plants. Unlike green algaes, few aquatic creatures have evolved to eat black algae but the Siamese Flying Fox Crossocheilus siamensis (also known as the SAE or Siamese Algae Eater) can be used as a means of control. The key to beating this, or any other nuisance algae, is to change the conditions in the aquarium with cause it to thrive.

With their bright colours and compact size, freshwater crustaceans have quickly become a popular addition to the aquarium hobby. Many of the varieties appearing in our stores are line-bred forms of less glamorous wild species and as such, will crossbreed and produce less attractive offspring if mixed. As with many fish species, the perfect solution is to pick your favourites and keep a large number! Given their tiny size, a growing number of shrimp-fans are finding it easy to house a number of small aquaria for their expanding collection.

With few exceptions, the popular shrimp species are Asian in origin and a growing number of amazing animals are being found in the Malili lakes of Sulawesi. These species are characterised by a need for alkaline water, which doesn't need to be particularly hard - a small tank of RO water powered by an air operated filter, or small internal power filter with a venturi outlet, will ensure pH and oxygen levels remain high. Stream dwelling species are more tolerant of lower pH systems and are very much at home in planted aquaria using CO2 where their appetite for algae and detritus makes them very useful. Many of these species are happy living at room temperature and will often fail to thrive in systems kept at high temperatures.

As invertebrates, many commonly used aquarium medications are toxic to shrimps (particularly those containing copper) and like their marine relatives, high levels of phosphate and nitrate can prove damaging and especially problematic when animals are moulting. A need for calcium can be met by providing your pets with foods made specifically for them, although they will happily eat most fish foods.

Never forget that your shrimps may be a welcome treat for a number of community fishes such as loaches and dwarf cichlids and make sure that they remain unmolested. For best results, tailor your fish selection to the needs of your shrimps, or enjoy them in a 'desk-top' style nano tank where they can tank centre stage.