Some so-called aquarium fish ' most notoriously the iridescent shark Pangasius hypophthalmus ' may look cute when they're babies, but they grow to mammoth sizes and can only be kept properly in public aquaria. Besides iridescent sharks, among the fishes best avoided are things like the sucking loach Gyrinocheilus aymonieri, Pacu, red-tailed catfish, Mbu puffers, barramundi, and tilapias; all splendid fish in their way, but far too large for most aquarists to maintain easily. But there are plenty of other things that you probably want to think twice about before spending any money on...

Non-aquatic plants

Most of the plants sold to aquarists are able to live underwater permanently. Some of these plants naturally live completely submerged their entire lives, such as Vallisneria spiralis, but others are only submerged seasonally, for example Echinodorus bleheri will spend many months of the year above the waterline in the waterlogged soil along the banks of rivers and lakes. But some of the plants sold to aquarists cannot and will not adapt to life in an aquarium. Most of these are simply repackaged houseplants, but a few a marsh plants that can survive a few months underwater. The houseplants obviously have no value to fishkeepers, but the marsh plants can be used in open-topped aquaria or vivaria where their roots can be kept underwater but their leaves are exposed to the air. Whether houseplant or marsh plant, kept permanently submerged these species will inevitably die and rot away, so however inexpensive they may be, they're still a waste of money.

Dracaena marginata is probably the most widely sold non-aquatic plant. As a houseplant, this species is known as a dragon tree and can reach up to 5 metres in height. The baby plants sold in aquarium shops are obviously much smaller, and typically comprise a single stiff stem bearing a rosette of long, narrow leaves that are frequently variegated and may be coloured green or red. The green dracaena Dracaena deremensis is similar but has leaves with undulating edges. Neither lasts long in an aquarium, and in fact both prefer relatively dry soil conditions. The lucky bamboo Dracaena sanderiana is equally unsuited to aquarium life.

Sold as stardust plants, species of Syngonium, particularly Syngonium podophyllum and Syngonium albolineatum, are equally widely traded. With their bright green colour and large, arrow-shaped leaves they are certainly attractive, but they will not last long in aquaria. Usually sold as the wheat planty is Chlorophytum bichetii, a close relative of the ubiquitous spider plant Chlorophytum comosum. It has a similar shape to the spider plant but can be identified by the presence of grape-like structures around the base of the plant. It is a marsh plant that does well in vivaria, but it has no place in an aquarium. Other houseplants on the non-aquatic roster are Fittonia verschaffeltii, recognised by its red or green leaves marked with a silvery-white network of veins, and the Malayan swordplant Aglaonema simplex with large dark green leaves borne on long, stiff stems. The Malayan swordplant is a hardy, long-lived species perfectly ideal for the open-topped aquarium but kept completely submerged its lifespan is rather short.

The aquatic palm Cyperus alternifolius often seen in aquarium shops is in fact neither a true aquatic plant nor a palm. It is in fact an amphibious sedge, and like the Malayan swordplant does very well in vivaria or open-topped aquaria where its leaves are above the waterline. But kept submerged it will surely die. The same goes for the very widely traded umbrella fern Selaginella willdenowii, easily recognised by its thin, woody stems and iridescent bluish-green Christmas tree-like foliage. This fern doesn't even live in marshes, being found on dry land in forests! The aquatic fern Trichomanes javanicum is at least a good marsh plant and will do well in a humid vivarium, but as an aquarium plant it is of no use.

Pond plants are sometimes offered as aquarium plants, and while some adapt nicely to coldwater tanks, the elevated temperatures in tropical aquaria often prove too much for them. Acorus gramineus is the classic example of this: hardy and low-maintenance in a pond where its leaves can grow above the waterline, but difficult to keep permanently submerged in a tropical aquarium. Other pond plants of questionable value to aquarists include some of the hairgrasses, Eleocharis spp., water chestnuts, and Canadian pondweed.

Table 1: Non-aquatic plants often sold in aquarium shops.

Potions and powders

Until the 1970s, there was a widespread belief in fishkeeping that old water was better than new water, and tonic salt and carbon were both used to reduce the need for water changes. Tonic salt (non-iodised table salt) reduces the toxicity of nitrate, and since many popular aquarium fish such as angelfish and mollies are particularly intolerant of high nitrate concentrations, the addition of tonic salt made it possible to keep those fishes safely without having to perform regular water changes. Although the more saline the water the less toxic the nitrate becomes, most freshwater fish cannot tolerate brackish water, so only a small amount could be used. In many old aquarium books the dosage suggested was around a teaspoon per gallon, and this is the sort of recommendation still sometimes heard in fishkeeping circles. However, modern fishkeeping stresses the need for large and regular water changes rather than relying on any magical properties old water might have, and for this reason adding salt to a freshwater aquarium is pointless. Moreover, salt can be positively harmful to certain aquarium fish. Salt has been implicated as a causative factor behind such mysterious illnesses as Malawi Bloat for example. Some aquarists maintain tonic salt is useful for tanks containing livebearers, although in the case of mollies this is almost certainly because mollies do not tolerate high levels of nitrate such as those found in poorly maintained or overstocked aquaria. If the pH and hardness need to be elevated, then calcareous substrates and filter media are far more effective than tonic salt, and brackish water fish actually need marine salt mix, not tonic salt.

Activated carbon (also known as activated charcoal) was another solution to the problem of how to avoid water changes. Carbon removes a variety of substances from water, including organic compounds that accumulate over time and cause the water to turn yellow. Protein skimmers in brackish and saltwater aquaria remove these substances directly, and water changes dilute them, so in modern fishkeeping the problem of yellowing water no longer exists, rendering carbon largely redundant. Carbon will also remove medications, which is obviously a bad thing when treating fish. Arguably, the only time carbon serves a purpose is when the aquarist needs to mop up medication before installing species such as shrimps, Mormyrids, and catfish that are intolerant of the widely used copper-based medications.

Though certainly not obsolete, the usefulness of algae and snail removing medications is very limited. Their main problem is that while they kill their targets effectively, they do nothing to remove their remains. In large quantities, decaying algae and snails can cause serious water quality problems. Burrowing snails in particular will not only decay but possibly decay anaerobically if they die deep inside the substrate, potentially producing highly toxic hydrogen sulphide in the process. Algae is invariably best dealt with holistically: by removing the tank from direct sunlight, adding fast-growing plants, performing regular water changes, and possibly by adding algae-eating fish. Similarly, rather than killing off the snails once they are there, it is far better to prevent them getting into the tank in the first place. An established snail population can be reduced by using snail traps and restricting the amount of available food by siphoning out organic waste and removing dead plant material at once. Many fish will eat snails too, so adding a pufferfish or loach to a tank will usually keep the snails under control.

Creepy-crawlies

Aquarists keeping freshwater fish are often tempted to add critters of various kinds. But while there are plenty of freshwater invertebrates ' including sponges and jellyfish ' remarkably few make good additions to freshwater aquaria. Insects are the predominant invertebrates in most freshwater habitats, but none has become established in the hobby. The problem is that while the smaller insects make good food for many fishes, the large insects are likely to turn the tables and eat your fish. Dragonfly larvae and water beetles will happily catch and eat things like Neons and guppies.

By contrast, the crustaceans are rather widely sold as oddball additions to freshwater tanks. Most of them are lively, relatively hardy, and generally easy to keep and feed. The small shrimps are particularly good aquarium residents, being fun to watch and often quite hard working algae eaters. On the other hand, the larger long-arm shrimps (Macrobrachium spp.) are territorial and potentially predatory, and consequently much less useful in community tanks. They are also expert diggers and quite likely to uproot delicate aquarium plants. Crayfish are similar to long-arm shrimps in temperament, and best kept in their own tank rather than with community fish. Worst of all are the crabs, none of which can be recommended for the average fishkeeper. Almost all the species traded are amphibious rather than aquatic, and this means that they need an environment with both water and dry land. While a few crabs are strictly herbivorous (the fiddler crabs for example) most are omnivores and will catch and eat small fish if they can. Crabs may also be demanding about water chemistry, the fiddler and red-claw crabs in particular being brackish water, not freshwater, denizens. Though fun in their own vivarium, crabs really have no place in the standard community tank.

The mollusks are a mixed bag as far as the freshwater aquarist goes. Snails can and do prosper in aquaria, and as noted earlier if conditions are favourable their populations can grow to pest proportions. Some snails will eat aquatic plants, but they are otherwise harmless scavengers that seem to tolerate poor water quality and the use of copper-based medications far better than any other freshwater invertebrates. However, not all aquarists want snails in their tanks. Several species are notorious for demolishing live plants. The Colombian Ramshorn is one of the worst in this regard, being not only able to eat quite robust species of plant like Amazon swords and Vallisneria, but also rather large and consequently that bit more destructive than the smaller species. But they are attractive animals with their Ramshorn-like shells and bright yellow and brown colouration, and for that reason very widely sold. Apple snails are not quite so destructive, and some aquarists have managed to keep them in planted tanks by providing alternative, softer fare such as blanched lettuce, but even so, mixing them with live plants is a gamble at best. The same goes for many of the generic pond snails that sometimes get sold as pets but more often hitchhike their way into the tank on aquarium plants.

Malayan livebearing snails have an ambiguous reputation according to many aquarists. On the one hand, they are totally harmless towards fish and plants, and also do an excellent job of cleaning the substrate while they burrow through it. But on the other hand they can breed very rapidly, and in a dirty aquarium with lots of decaying plant material and excess food, a few of these snails can become hundreds of them in a remarkably short period of time. Altogether safer as far as numbers go are the Nerites, which rarely breed in freshwater aquaria. Most of the common species in the trade are actually brackish water snails, and won't breed unless the salinity reaches a certain level. Nerites are just as harmless towards fish and plants as Malayan livebearing snails, and make exceptionally good algae eaters, taking not just green algae but also diatoms, and in the case of the zebra Nerite, blue-green algae as well.

While snails are often quite serviceable additions to the freshwater aquarium, the Asian clams (typically Corbicula fluminea) and various types of swan mussels offered for sale are almost always a waste of money. They are exclusively filter-feeding animals, and without regular feeding eventually starve to death. Commercially-prepared filter feeder foods are available to marine aquarists for use with things like corals and tubeworms, and these work well with freshwater clams and mussels too, so the potential is there to keep them alive. But there are additional problems to workaround even if the aquarist is prepared to feed their clams and mussels with such foods on a daily basis. The mussels in particular are coldwater animals, and die in tropical tanks relatively quickly, presumably from heat exhaustion or the absence of sufficient oxygen in the water. Clams and mussels also like to dig into the substrate, and beyond not being on view much of the time, this can also be disruptive as they uproot aquarium plants. Worse still, if a clam or mussel dies hidden away somewhere, the first the aquarist is likely to know about it is when the water quality crashes. All in all, clams and mussels are best avoided.

Table 2: Snail species offered to aquarists

Read up first!

Besides the living things discussed here, the aquarist also needs to look out for the inanimate objects that may be unsuitable for their aquarium. Seashells, barnacles, and bleached corals are often on sale and can make interesting additions to certain tanks, but because these are calcareous, they may harden the water and raise the pH as well. Though this is not a problem in tanks where the water is already hard and alkaline, in the average community aquarium containing soft water fish adding anything that hardens the water and raises the pH is not a good idea. Similarly rocks and substrates such as limestone and coral sand may need to be eschewed in favour of chemically inert ones like granite and gravel.

All these choices highlight the need for aquarists to do their research before spending their money. Supplementing an old aquarium book picked up at the junk shop with one explaining the state of the art should dispel any uncertainty over the usefulness of salt and carbon in the freshwater aquarium, and investing in books about aquarium plants and fish will help the shopper identify the stock on sale at your local tropical fish shop. Whether you end up stuck with a metre-long catfish or simply a tank load of rotting plants, problems can be avoided by making sure you know exactly what you're buying before handing over any money. There is a bigger question why so many aquarium shops continue to sell things largely or entirely unsuitable to aquarium life (non-aquatic plants for example) and the answer is probably partly that demand for them exists from novice fishkeepers and partly that the retailers don't know any better. All you can do is do your research and spend your money wisely.