Take it from the top; 

To my mind, there's nothing better than a busy community tank. Most aquarists understand the difference between the species that swim in midwater and those that stay close to the bottom, with angelfish being of the first sort and Corydoras catfish the second. However, I'm often surprised that people don't apply this same understanding to the top of the tank. Just as there are fishes adapted to feeding off the substrate at the bottom of the tank, so too are there species that rarely stray far from the surface of the water. These fishes are invariably predators of one sort or another, most often taking the larval forms of insects such as midges and mosquitoes. 

In my opinion, if you really want to get the most out of your community tank, you need to divide up your livestock into fish for the top, fish for the middle, and fish for the bottom. Assuming you're using species of similar size, a good rule of thumb is for every two midwater fish choose one fish for the top and another for the bottom. So, a school of six bleeding heart tetras, Hyphessobrycon erythrostigma, would be perfectly complemented by three Corydoras catfish and three hatchetfish, Gasteropelecus spp.

Surface dwelling fish often exhibit some important similarities regardless of the taxonomic groups to which they belong. This 'convergent evolution' shows that life at the top of a stream or river imposes certain constraints on these fish that they need to accommodate if they are to survive. With few exceptions, these fish have upwards-pointing mouths, all the better for snatching prey from the surface of the water. Some of these prey animals live just underneath the waterline, as is the case with mosquito larvae, but these fish will also take insects that have landed on top of the water, whether accidentally or deliberately. To help them spot their prey, these fish invariably have eyes close to the top of their heads. 

Living close to the surface of the water allows these fish good access to a supply of food that bottom dwelling species can only wish for, but it also places them at far greater risk from predators like birds. Whereas mid- and bottom water fish can take advantage of plants and caves as useful hiding places, fishes at the surface have fewer places to hide. Their eyes help them spot danger coming, and to swim away to safety these fish very often show adaptations that allow them to accelerate very rapidly. In particular, these fish usually have their dorsal and anal fins close by the tail fin, a configuration that maximises their ability to build up speed quickly. Midwater fish, by contrast, usually have their fins spread out more evenly, a better arrangement for maintaining high speed or efficiency but not so good for sheer acceleration. 

One final characteristic of surface dwelling fish is a willingness to jump, or even to fly. Living close to the surface makes them obvious to larger fish swimming beneath them, and simply swimming away rapidly might not be enough to avoid predators coming up from underneath. Many species avoid such predators by leaping out of the water, effectively vanishing from the view of their pursuers, and thereby giving them a good chance to find safety. Halfbeaks, archerfish, butterflyfish, and four-eyed fish are all remarkably capable jumpers, but few freshwater fish match the prowess of hatchetfish when it comes to aerial manoeuvres. These amazing fish are able to extend their leaps by flapping their greatly enlarged pectoral fins, and specimens as small as 6 cm (2.5 inches) are said to be able to fly over 2 meters (6 feet), more than enough to avoid most aquatic predators.

Compared with most other fish, surface dwellers aren't at all demanding when it comes to aquarium life. Because they have evolved to occupy an open, unfussy environment, they don't care very much about plants, substrate, or caves. With few exceptions, these fish are remarkably non-territorial, and generally get along very well with similar sized tankmates. Conversely, in a reasonably deep tank, territorial bottom dwellers like dwarf cichlids simply ignore the fishes at the very top of the tank. Indeed many bottom dwelling fish are noticeably more relaxed and outgoing when kept in tanks with at least a few surface dwelling species; by acting as 'dither fish', the surface dwelling fish reassure the fishes deeper down the tank that there are no predators about, so it's safe to come out and forage for food. 

Nonetheless, there are a few things to consider before buying surface dwelling fish. Perhaps the most important thing is that these fish shouldn't be kept in open-topped aquaria. Eventually, something will startle them, and they will leap out onto the carpet. While not the escape artists that spiny eels and ropefish can be, at the very least your aquarium needs to have a hood or condensation tray to 'bounce back' any free-flying fish. 

Most species appreciate a few floating plants, as well. Floating plants reassure these often nervous fishes that a safe hiding place isn't far away, and while few of these fish hide away like many bottom dwelling catfish, they still need to feel secure. Floating plants do have a reputation for being troublesome in aquaria though. Plants that grow beneath the waterline, like hornwort, Ceratophyllum demersum, are usually the easiest to accommodate. Hornwort does well in only moderately lit aquaria, is indifferent to water hardness or pH, and grows well but not so rapidly as to shade other species lower down in the tank. Crystalwort, Riccia fluitans, is another good floating plant that works well in most tanks. 

Both hornwort and crystalwort do need to be thinned periodically, but apart from this, present few problems. The same cannot be said for those floating plants that grow on top of the water. Pistia stratiotes is widely sold, very pretty, and inexpensive, but unfortunately is difficult to maintain in small aquaria. Its leaves invariably end up wedged between the water and the hood, where they are burned by the aquarium lights. Although smaller, Amazon frogbit, Limnobium laevigatum, and floating Salvinia, Salvinia auriculata, are just as demanding of a cool, airy space between the water and the lights.

Hatchetfish are the surface dwelling fish par excellence. Perfectly adapted for life at the top of the water column, these South American characins combine fine senses with high speed and a remarkable ability to fly away from danger. In aquaria, hatchetfish have proven to be peaceful and relatively hardy, and they make very good community tank residents. Two species are traded regularly and are easy enough to find, but it has to be admitted that their lack of bright colours has tended to diminish their attractiveness when compared with more brightly coloured characins. 

The two common species are the marbled hatchet, Carnegiella marthae, and the silver hatchet, Gasteropelecus sternicula. The marbled hatchet is a tiny fish, barely 4 cm in length, and so is best suited to tanks containing other small species. Neons, cardinals, whiptail catfish, Corydoras, and Pencilfish would all make ideal companions for a community tank containing only South American species. Otherwise, these little hatchets could equally easily be combined with dwarf barbs and rasboras, the smaller gouramis, and kuhli loaches, all of which would thrive in the soft, acid water that marbled hatchets seem to do best in. Considerably larger, the silver hatchetfish will work well with peaceful species as large as pearl gouramis, dwarf cichlids, angelfish, and plecs. Silver hatchetfish are tolerant, even boisterous, fish when kept properly, and quickly settle down to become active and entertaining characters in any community that they kept in, making up for their lack of colour by sheer presence and personality. My specimens became tame enough to hand feed within days, and they even nip at the hairs on my arm if I'm working in the tank! 

There are a few other species of hatchetfish traded from time to time, and apart from differences in size and colour, they are all very similar in requirements and behaviour. Perhaps the star among the rarely seen species is the spotted hatchetfish, Gasteropelecus maculatus, which grows to no less than 9 cm (3.5 inches) in length. While big enough to eat very small fish such as livebearer fry, it is otherwise just as peaceful as its smaller cousins, and makes a superb addition to a soft water community tank. 

The African equivalent to the hatchetfish is perhaps the butterflyfish, Pantodon buchholzi. Closely related to the Arowanas, this predatory species cannot be kept with smaller fish like guppies or neons, but alongside companions of similar size, it makes an excellent community fish. Individuals will squabble amongst themselves for space, so they shouldn't be overcrowded: two or three specimens will coexist in a 180-litre (30-gallon) tank provided there are plenty of floating plants. On the other hand, they ignore completely any fish swimming in the middle or lower levels of the aquarium. An aquarist wanting to maintain an African theme could very effectively create a community putting this species alongside Congo tetras, ropefish, climbing perch, and kribensis.  

Most killifish fare best in single species aquaria, but a few work well in community tanks. Easily the most widely traded of these are the Asian killies, Aplocheilus spp. Marketed under a variety of names including 'Ceylon panchax' and 'golden wonder', Aplocheilus lineatus is probably the most regularly seen species and certainly the most accommodating as far as water requirements go. It will do well in water conditions from soft and acidic through to slightly brackish, making it a versatile community species. Be warned though, this pike-like fish is every inch a predator, so you need to choose tankmates with care. Neons, guppies, and other small fish will simply vanish one at a time, but things like Corydoras catfish, medium sized barbs, dwarf cichlids, and gouramis will all coexist amicably with this species. 

Closely related to the Asian killifish is the golden or jewelled panchax, Pachypanchax playfairi. As its common names suggest, this is a most attractively coloured fish, and it is a shame that it isn't as widely traded now as they were a few decades ago. Nonetheless, this is a fish worth tracking down, as good specimens can be truly stunning fish, having a bright yellow ground colour with red, green, and blue speckles all along the body. Like the Asian killies, these fish are largely indifferent to water chemistry, but they do not like very soft, acid water. Golden panchax are easy to keep, usually peaceful fish, but do bear in mind that they can be predatory and may eat very small tankmates. Males may be territorial, so avoid overcrowding them. One strange characteristic of this fish is its ability to raise its scales, giving a dropsy-like appearance. These fish most often do this when in spawning condition. 

At the other end of the size spectrum is the dwarf Mosquitofish, a superb surface dwelling fish for the hardwater aquarium. In shape and habit, these fish resemble guppies, and while they lack the gaudy colours of guppies, they are attractive fishes nonetheless. Males are tiny, less than 2 cm (1 inch) in length, and females are only a little bigger. In the wild, these fish inhabit thickly vegetated ditches and streams, and it is fair to say that the more floating plants you have in your aquarium, the happier these fish will be. If you're after a fish for a mini-aquarium with less than 30 litres (7 gallons) of water, the dwarf Mosquitofish is an excellent choice. Choosing tankmates can be problematical given its size, but in a small, slightly brackish water aquarium very suitable tankmates would include bumblebee gobies (Brachygobius spp.) and Amano shrimps (Caridina spp.) both of which thrive in such conditions. 

More interesting livebearers comes from the halfbeak family, of which several species are sold regularly and inexpensively. While most halfbeaks are adaptable and peaceful, the silver halfbeak stands out as one of the most durable species available. Apparently a species of Dermogenys, these fish differ from the most familiar wrestling halfbeaks in lacking red and yellow markings on their fins. They are also notable for not needing salt in their water, making them easy to accommodate in the average community tank. This halfbeak barely reaches 7 cm (3 inches) in length, and so is eminently suitable for inclusion in small aquaria, but like all halfbeaks, it can be fiercely territorial, so take care not to overcrowd the males.

Rounding off my look at the surface dwellers is a much-misunderstood fish, the Siamese fighter. Males are sold for use in tiny, one-fish tanks almost likely living ornaments and this disguises the fact that these fish can be excellent community fish. To start with, males are only aggressive towards one another, and are very peaceful towards other species. Siamese fighting fish rarely stray far from the surface of the aquarium, and in particular like to stay well hidden among the floating plants. Even so, their bright colours can add a wonderful touch to the community tank. The red and blue specimens in particular contribute a richness hardly matched by any other freshwater fish. 

So why don't we see Siamese fighters in community tanks more often? Bottom line, they aren't easy fish, despite their ubiquity. Male Siamese fighters are usually raised alone since they will fight with one another, so unlike most other farmed aquarium fish, they have no experience of living in communities with other individuals. This makes them shy, slow at feeding time, and often easily bullied by more aggressive tankmates. Females generally fare somewhat better, having been raised together in large groups, but they are still relatively slow moving fish that will not do well with nippy, aggressive tankmates like barbs or cichlids. Examples of fish that Siamese fighters of both sexes will get along well with include neons, Corydoras, hatchetfish, Pencilfish, and glass catfish, to name but a few. 

Our strange love affair with the Siamese fighter has turned a durable little fish into a delicate, if handsome creature often unable to survive the hurly-burly of the community tank. Seeing them explore a thickly planted community tank is an unexpected delight though, and a reminder that life at the top can be a struggle, but is definitely worth it!

Marbled hatchetfish 

Latin name:                   Carnegiella strigata

Size:                              4 cm (1.5 inches)

Water requirements:    Soft, acid water preferred, but adaptable

Food:                             Flake, frozen bloodworms, small insects

Social behaviour:          Peaceful, schooling fish

Breeding:                      Egg-layer, similar to other South American characins

Common silver hatchetfish 

Latin name:                   Gasteropelecus sternicula

Size:                              6.5 cm (2.5 inches)

Water requirements:    Soft, acid water preferred, but adaptable

Food:                             Flake, frozen bloodworms, small insects

Social behaviour:          Peaceful, schooling fish

Breeding:                      Not yet bred in captivity

African butterflyfish 

Latin name:                   Pantodon buchholzi

Size:                              10 cm (4 inches)

Water requirements:    Soft, acid water preferred, but adaptable

Food:                             Live foods (including insects and small fish) preferred, but will take frozen foods and, eventually, flake

Social behaviour:          Territorial amongst themselves

Breeding:                      Egg-layer, rather difficult to breed

Asian killifish 

Latin name:                   Aplocheilus spp.

Size:                              Up to 10 cm (4 inches)

Water requirements:    Adaptable, provided extremes are avoided

Food:                             Flake and frozen foods, as well as small fish and invertebrates

Social behaviour:          Territorial amongst themselves, but peaceful towards fish too big to be eaten

Breeding:                      Egg-layer, not difficult to spawn

Golden panchax 

Latin name:                   Pachypanchax playfairi

Size:                              Up to 10 cm (4 inches)

Water requirements:    Neutral to slightly hard and alkaline water preferred

Food:                             Flake and frozen foods, as well as small fish and invertebrates

Social behaviour:          Territorial amongst themselves, but peaceful towards fish too big to be eaten

Breeding:                      Egg-layer, not difficult to spawn

Dwarf Mosquitofish 

Latin name:                   Heterandria formosa

Size:                              Females around 4 cm (1.5 inches), males smaller

Water requirements:    Hard, alkaline water preferred, also does well in slightly brackish (SG up to 1.005)

Food:                             Flakes, algae, small live foods such as Daphnia

Social behaviour:          Males may squabble, but essentially these fish are peaceful and easy to keep

Breeding:                      Livebearers and easy to breed

Silver halfbeak 

Latin name:                   Dermogenys sp.

Size:                              7 cm (3 inches)

Water requirements:    Soft, slightly acid water preferred but does well in hard, even slightly brackish, water as well

Food:                             Flake, bloodworms, small invertebrates

Social behaviour:          Males best kept one to a tank, but females are completely peaceful

Breeding:                      Livebearer, but can be difficult to breed nonetheless

Siamese fighting fish 

Latin name:                   Betta splendens

Size:                              6 cm (2 inches)

Water requirements:    Adaptable, provided extremes are avoided

Food:                             Any small flake, pellet, or frozen food

Social behaviour:          Males territorial, females much more peaceful

Breeding:                      Bubble-nest builder, male guards the eggs