Moving on

Armoured catfish of the family Loricariidae are literally being described faster than scientists can describe and name them, and so a shorthand system of identifying these catfish has come into use, the L-numbers, where 'L' stands for 'Loricariidae'. Not all the armoured catfish have L-numbers; species that have been recognised for decades, like the common Plec Hypostomus plecostomus, don't have L-numbers, and neither do catfish belonging to other families, such as Corydoras. 

Even after a catfish receives its proper scientific name, many aquarists continue to refer to them by their L-number. So the zebra Plec, Hypancistrus zebra, is L046, and the sailfin Plec, Glyptoperichthys gibbiceps, is L083. But L-numbers are used most commonly for the species that are new to the hobby. The gold nugget Plec, Baryancistrus sp., for example, is L018, and the 'watermelon' and 'olive' varieties of the royal Plec, Panaque nigrolineatus, which may or may not be distinct species, are L027a and L027b respectively. There is a good deal of variation among the L-number catfish, in particular with regard to size, with some being dwarfs comparable to the popular Ancistrus catfish and others being giants that can exceed lengths of 60 cm (24 inches). 

L-number catfish can be a great addition to the community tank because for the most part they share many of the characteristics of the more familiar Plecs. They are generally peaceful omnivores that do not pose any kind of threat to other fish in the tank. Some are notably more demanding in terms of water quality, but others are robust and hardy. Almost all Loricariid catfish are territorial, which usually translates to keeping one to a tank, though smaller species can be kept in groups provided the aquarium is large enough, and breeding them is a distinct possibility. 

Just as with the more common varieties of Plec and whiptail catfish, these fish are to a large degree herbivorous, and green algae, blanched lettuce, peas, and slices of zucchini being considered good staple foods. In addition, many readily take small meaty foods such as shrimps and bloodworms, as well as regular catfish pellets. Some species also eat wood, in which case a piece of bogwood should be placed in the aquarium for them to graze on. 

Buying an L-number catfish isn't difficult, with many aquarium stores regularly keeping the more attractive varieties in stock all year round. Many are inexpensive, but some can cost more than marines, so in part your choice will be dictated by budgetary considerations. One thing to look out for is how well these fish in the store are eating: with many species, if they lose a certain amount of body mass, they never seem to recover and eventually die. Only buy specimens that look plump and healthy.

Some aquarists like to 'collect' as many species as they can from certain taxonomic groups, with catfish and cichlids being perhaps the most popular of this sort of specialty. Of course you don't have to exclude other types of fish from aquaria including these fish, and many catfish fanciers, for example, keep their favoured fishes in tanks alongside barbs and characins. But where the average community tank contains fish chosen purely on aesthetic grounds, in aquaria with a taxonomic theme the aim is to chose different fish from one group that have complementary ecological niches. So a catfish-themed tank might contain a algae-eating Plec, some midwater predators like Pimelodus pictus, a school of scavenging Corydoras catfish, and a few Synodontis nigriventris scuttling underneath some floating plants. Provided you had the space, there's no reason you couldn't add a school of tiger barbs or Congo tetras to this mix as well, but even without them this would be a lively aquarium that would be a lot of fun to watch. 

Aquarists keeping the African Rift Valley cichlids are pretty much keeping fish from a single taxonomic group by default, even allowing for the odd Synodontis or Plec that is pressed into service as housekeeper. But even cichlids from different parts of the world can be mixed successfully. Domesticated varieties of angelfish (originally from South America), Firemouth cichlids (from Central America), Kribensis (from West Africa), and orange Chromides (from Asia) will all do well in neutral to moderately hard, alkaline water and in a large aquarium will coexist peacefully. Each has a unique shape and colour, and with angelfish swimming in midwater and the others staying close to the substrate they would fill out the space in the tank nicely. Rainbowfish and livebearers would be great choices for the top of the tank, adding some colour and activity. 

Other good specialisations include livebearers, gobies, and killifish. These fish are generally small, making them especially good choices for aquarists with limited space at their disposal. They are also generally quite hardy provided their basic water requirements are met, with some species needing soft water and others slightly brackish. The air-breathing labyrinth fish offer another great option, with wild Bettas, climbing perch, and the more unusual gouramis (such as croaking gouramis and honey gouramis) all rewarding subjects for single-species or carefully constructed community tanks.

Aquaria based around particular biotopes are challenging but popular. Instead of simply collecting a bunch of fish that happen to interest you, the deal here is to assemble a community that reflects a particular place or habitat. This includes not just the fish but also the plants, and if you're a stickler for detail you'll want to use decorative materials such as rocks and sand that mimic those of the place in question. 

An aquarium based around a fast flowing stream in Africa might therefore include Congo tetras, African glass catfish, and blockhead cichlids (Steatocranus casuarius), all fish that enjoy this sort of habitat. Smooth pebbles and boulders would be the primary decorative materials, and are far more suggestive of fast flowing water than, say, bogwood or jagged rocks like slate. Fine gravel or silica sand would work well as a substrate and look more realistic than coloured gravel, coral sand, or peat. Finally, instead of planting thickly with the more familiar species of Echinodorus or Vallisneria, you would opt for African plants such as Anubias and Bolbitis. Such a design would lend itself to a long but shallow tank where the plants would be able to grow above the water level and the filter could be set up to create lots of turbulence at the surface. 

You don't necessarily need to stick to a single geographical location for this to work well, and many aquarists find that re-creating a certain habitat using fish and plants from around the world is quite effective. Marine reef aquaria obviously fall into this category, with many people keeping mixtures of fish and invertebrates from the Indo-Pacific, the Red Sea, and the Caribbean without problems. Blackwater habitats are another good example of this, and work well with fish such as acara (from South America), clown loaches (from South East Asia), and Mormyrids (from Africa) all appreciate deep, dark tanks and should get on well together if given enough space. At a smaller scale, white cloud mountain minnows (from China), Danios (from India and South East Asia), Rainbowfish (from Australia and New Guinea), and rummy nose tetras (from South America) all appreciate shallow, brightly lit aquaria with a strong water current and could easily be kept together as a 'jungle stream' community.

A popular specialisation among aquarists interested in re-creating a single biotope is the brackish water aquarium. Something of a middle ground between freshwater and marine aquaria, in the past brackish water aquaria were viewed primarily as a stepping stone from the easier freshwater side of the hobby to keeping the more delicate coral reef fish and invertebrates. Because many brackish water fish will adapt to sea water without problems, the idea was that you could gradually increase the salinity of a brackish water aquarium over a few weeks or months and thus simultaneously mature the biological filter, removing the need to risk losing any expensive marine fish. Once this was done, you could either keep your brackish water fish alongside your marines, or else pass them along to another aquarist. 

But in recent years brackish water aquaria have become a end in themselves, primarily because listed among the brackish water fish are some of the most interesting species kept by hobbyists. These include archerfish, mudskippers, flatfish, pipefish, and of course the ever popular pufferfish. There are also lots of little jewels -- small, attractive fish that adapt well to captivity and do not require a lot of space. Among these are the glassfish, halfbeaks, and lots of different kinds of goby and livebearer. 

Essentially a brackish water tank is freshwater aquarium with a bit of salt added, and the additional hardware demanded by marine fishkeepers, such as protein skimmers and ultraviolet filtration, is generally not necessary. There are some problems with plants though, many species not doing well in salty water, but most of the hardier species of Echinodorus, Hygrophila, and Vallisneria, among others, do just fine provided the salinity is not too high. One new concept aquarists interested in brackish water fish do need to learn about is 'specific gravity', a proxy for salinity and measured using a hydrometer. Freshwater has a specific gravity (or SG) of 1, whereas tropical marine water has an SG of around 1.024. Technically brackish water is anywhere between these two extremes, but in practise most brackish water aquaria are maintained at around 1.002 to 1.005, although adults of a few species, most notably monos, scats, and Colombian shark catfish, may require a higher specific gravity, perhaps even fully marine conditions.

It is always something of a surprise to newcomers to the hobby to discover that big, predatory fish are not necessarily aggressive or difficult to keep. In fact once you get past the fact that the need a lot of space and cannot be kept with smaller fish, many are actually quite docile and may even become tame enough to be hand fed. Oscars and some of the big South American catfish are classic examples of this, and very often they manage to acquire something of the status of a true family pet, interacting with the other people in the home to a degree rarely associated with fish in general. 

Big, predatory fish also tend to be rather impressive animals, and many are downright stunning to look at. Colombian shark catfish, garpike, jaguar cichlids, and freshwater stingrays have to be among the best looking fish kept by hobbyists, and it is no wonder that they attract the eye of the discerning fishkeeper. While they may not be as brightly coloured as neon tetras, platies, or goldfish, these fish clearly look powerful and they move with a purposeful grace many smaller fish lack. Many are also very observant, watching the world outside the tank as well as within it, and respond to their owners as much as any large fish kept with them. 

Having laid out the good points, it's fair to take a look at the problems as well. First off, big fish need big tanks with big filters. Even an aquarium that would be considered quite sizeable by beginners (such as a 120 litre / 30 US gallon tank) is far too small for even one of these tankbusters, and something of the order of 700 litres / 180 US gallons is much more realistic. If setting aside this sort of space for an aquarium doesn't appeal to you, then forget about keeping big fish.

The final, and perhaps most rewarding, avenue in the fishkeeping hobby to be discussed here is fish breeding. There is definitely something special about being able to see the entire life cycle of a fish in an aquarium, and among aquarists fish breeders are something of an elite. Sure, anyone can have guppies breed in their community tank, but actually breeding distinct varieties of even fish like guppies to a show standard is much more difficult, and creating your own varieties is even more challenging. 

Livebearers are a good place to start though, primarily because the newly born fish tend to be rather large and so are able to handle quite sizeable foods, even flake foods, right away. On the other hand, once you get away from the 'big four' -- guppies, platies, swordtails, and mollies -- simply getting many livebearers to breed at all is far from easy. Some species, such as halfbeaks, represent a challenge every bit as stiff as breeding egg layers. 

Cichlids are of course among the most popular subjects for the dedicated fish breeder. For one thing, many cichlids make exceptionally good parents, and will take care of many of the important chores like removing fungused eggs without any help from the aquarist. Some will also make sure the newly hatched fry get fed properly as well. Kribensis, for example, seem well able to lead their batch of babies from one patch of algae and detritus to another at the same time as shooing away any potential predators that come too close. Undoubtedly this is one reason that Kribs are so good at breeding even in community tanks. Other cichlids, like angels, tend to require a bit more help, and more often than not you will need to remove the eggs from the parents so that you look after them yourself. This isn't particularly difficult, but before embarking on breeding these fish it would be well to read up on the subject first. 

Killifish, catfish, gobies, and many other egg laying fish are routinely bred in captivity but usually require much more work than cichlids. In some cases, this is because the fry are small and require consequently tiny live foods. Gobies and labyrinth fish are notorious in this regard, and the first foods may well include things like 'infusoria' (cultures of single-celled organisms), rotifers, and drops of algae-rich water. 

But with other fish it isn't so much raising the fry as getting the parents to spawn in the first place that is the challenge. Barbs, characins, and killifish, for example, may well thrive in hard, alkaline water, but they won't breed; instead, you'll need to keep them in a tank with soft, acidic water and then employ just the right triggers (such as a change in temperature) to get them to spawn. It is often a good idea to concentrate on species that are well suited to your local water conditions. So if, for example, your local water is hard and alkaline, you'd be much better off concentrating on cichlids, gobies, or livebearers than barbs or killifish. 

To wrap up then, we've looked at six ways to extend your hobby from a community tank  into something more challenging. Whether it's re-creating a bit of the Rio Grande or raising the next generation of fancy guppies that appeals to you, I hope that I've inspired you to go buy a second (or third, or fourth'¦) fish tank so that you can try something new!