Security
Some research is needed, either through text books, web sites, newsgroups or aquarium clubs into the nature of the fish being considered. Their body shape, feeding habits and company they keep, all provide clues to their needs. Meeting their security requirements will result in calmer and healthier fish which will behave more representatively of their normal behaviour in nature.
Details...(alphabetically)
Quick picks African cichlids, Apistos & dwarf cichlids, Angelfish, Barbs, Bettas, Cichlids (other), Corys, Danios, Discus, Goldfish, Gouramis, Livebearers, Loaches, Piranha, Plecos and Tetras & Rasboras.
African cichlids
There are basically 3 types, utaka (open water fishes), mbuna (rock dwellers) and African dwarfs.
Utaka
Utaka do best with open water and long visible sight lines. They are prone to attacks from underneath, so they should not be forced down into hostile territories. They are generally not hostile (except within their own species for pecking order & spawning), but can, to an extent, adapt using their larger size. Should be kept in groups, 4 or more. Typical grouping is 1 male with 2 or 3 females, though this will vary.
Mbuna
These are a recipe, best established with a group of juveniles with plenty of shelter (stacked rockwork). Security is a stable pecking order and a cave to call their own. Neither of these things happen easily in an aquarium, as the pecking order is quite dynamic, and casualties may occur periodically. Mostly herbivorous, how well these fish survive in nature depends on how much rock they can protect (which are covered with the algae they graze on). Some species are omnivorous, specializing in bugs that they find in the rocks, so they are less territorial, moving to new location to find new bugs.
Security is in some measure achieved by over-crowding, with fish of similar size and temperament, and lots of shelter (but not enough to provide everyone their own home). This maintains a state of chaos. Ideally it's a low steady state of chaos, so we can say that an equilibrium has been achieved. Alpha males who spend energy directed at too many others, risk becoming a victim when tired out. However, much of the pecking order is determined by bravado, and strength is only used for battles.
This equilibrium or pecking order, is somewhat fragile. Besides being continuously challenged, as the fish mature, any deaths or new introductions are an invitation for wholesale changes in the pecking order. It is not uncommon to have a string of casulties immediately after a death or a new fish being added to the community.
African Dwarfs
Security requirements vary by species. Shell dwellers do best with calmer environments, with empty snail shells provided on a sand substrate. Many of the Neos and Julies (Julidos) can be extremely hostile within their species, but totally ignore other fish (except when spawning sites are being staked out). Note that not all Julies are dwarfs, but they and many of the Neos (ie: leleupis), are extremely fast darters, using their heads as effective torpedo-like battering rams, making them more than a match for slightly larger fish. Bricardis are sometimes in this class of fish, being small and appearing more delicate, their strength is in numbers and in organized attacks. Raising these fishes together from juveniles is usually essential, as is plenty of rockwork with varied sizes of caves.
Apistos & dwarf cichlids
Somewhat hostile to each other, especially in small tanks, and very hostile to others when protecting their spawns. They are susceptible to larger hostile fish. They will often ignore and be ignored by top and mid-feeders, preferring to stay in the underbrush, so it's important to provide good low and mid-ground coverage using a variety of rocks, driftwood and/or plants.
Angelfish & Discus
Short sight lines, lots of thin vertical plants, floating plant cover and no overly active fish (unless very small). Driftwood should be low or vertical (so the fish can easily manoeuvre through). Vertical pieces of slate useful to have. Avoid large bushy plants, excessive water turbulence and sharp or pointed edges on wood, rocks etc.
Barbs
Not overly demanding on environment for security. Barbs are non-specialized fish who work best in packs (and this is their source of security). Their front facing mouths allow them to feed effectively from the surface or the substrate, making them very flexible. When secure, barbs will tend to stay within their 'pack' and are less likely to annoy other fish (thankfully). This is an especially useful strategy to exploit with the more aggressive barbs such as Tigers.
Bettas
Floating plants and or debris such as driftwood. Shallow tanks are more representative of their natural environment, and are of importance when they are spawning. Bettas vary considerably in the willingness to attack or even defend themselves from hostilities. A secure environment would have no other Bettas in it. A large enough tank may hold several females with a single male. Small active nippy fish may torment a Betta to complete distraction and cause him to remain in hiding. Because of their bright coloration, fin length and slower rate of motion, Bettas are more typically victims than instigators when put into a mixed community tank. They are very carnivorous, and will provide crowd control for livebearers. Occasionally we come across one which is somewhat agoraphobic, so these would do best in a smaller tank (heated filtered 3-5g) with a lower activity level.
Cichlids (other)
Larger cichlids (including Angelfish and Discus) are more sensitive to tank height, preferring to be at eye level or higher to the pet-keeper (this is not uncommon with many animals). Mostly omnivores, many of these fish are highly carnivorous, and as such are more thoughtful about their actions. Carnivores need to weigh the cost/benefit risk ratio in deciding to hunt a meal, while herbivorous only need to swim to the correct environment and begin grazing. This is what separates the wolf from the sheep, or the fox from the chicken. These larger cichlids are very aware of the contents of their immediate environment, and generally consider everything they see outside of the tank as part of their unchallenged domain. Moving a rock in an Oscar tanks will often result in the Oscar lying flat on the substrate for several hours (showing his dissatisfaction with the change, or perhaps lying low to see if the change resulted in any new predators). The same thing can sometimes be achieved by re-arranging the furniture in front of an Oscar tank. These fish also get bored easily, not having the extra stimulus they need, so they will re-arrange the tank contents, moving all the gravel from one side to the other on a daily basis, pulling heaters off their base and sometimes breaking them, pulling hoses loose, ripping plants apart etc.
Security is achieved with non-threatening company they are familiar with (raised with), not being overcrowded, a variety of materials in the tank (toys) and having some ditherfish around.
Corys
Shoaling fish, should be kept in as large a group as possible (4 to 6). Different species may or may not shoal together. They do best in sand substrate or very smooth small gravel, where there is minimal competition for food once it has hit the bottom. May be problematic to keep with aggressive bottom-feeding fish (Angelfish, Discus and most other cichlids, Barbs etc), as their barbels get damaged as the other fish try to eat the same bottom-feeder pellets. Can also be difficult to keep well fed when housed with very aggressive top-feeders (use sinking bottom-feeder pellets into underbrush). Provide plenty of low foreground and midground plants, so the fish have some peace when eating.
Danios
These fish are fairly unconcerned about their environments, being gregarious and continuously curious about anything which moves. Provide a measure of security with plants which have bushy well spaced leaves, and keep them in large numbers.
Discus
see Angelfish
Goldfish
Not overly concerned about their environment, security is achieved by not having predators or over-aggressive goldfish around. Provide lots of room, use appropriate ground crew (Dojo loach, Apple snails), avoid overlarge bushy plants, avoid anything which requires them to manoeuvre much (they get stuck easily). There are 2 main varieties; short (Fantails, Orandas, Celestial, Lionfish, etc) and long (Comets, Shebunkin etc), and the short types should not be mixed with the long. This includes Koi which mix poorly with any of the Goldfish except for Comets and Shebunkins. Shape also determines the best dimensions for their tanks. Many are particularly vulnerable to attacks, and special care is required (Oranda, Globe-eye, PomPom etc) to keep them away from fish which could attack their special features.
Gouramis
Floating plants and or debris such as driftwood. There is significant variation in the family of Anabantidaes (which includes Paradise fish and Bettas), so I'll only cover the most common in the trade. Larger semi-aggressive gouramis (Gold, Opaline, 3-spot) need lots of room, and are basically treated like cichlids. Their typical tormentors are each other and small very aggressive fish. Medium sized gouramis (starting with the Moonlight and Pearl gourami, Thicklipped and then Dwarf gouramis) do fine in smaller tanks, but with some discretion. They are less likely to torment each other, but it's not unheard of. These fish have a more docile nature and should not be with semi or aggressive fish. Delicate gouramis (Chocolate, Licorice and Croaking) should be kept in a very low key environment, with plenty of plants, small community fish and no predators.
Livebearers
Not particularly demanding on environment, should be kept away from predators and carnivores.
Loaches
Security is in packs and with shelters which they can pile into, under cover, away from bright lights. Loaches don't have a lot of fish predators, so their environment and their pack size are the consideration.
Piranha
A timid fish which has a tendency to eat its tank-mates, piranhas are a bit difficult to provide a secure environment for. Floating and thin plants (Frogbit & Vallesneria) will provide an appropriate habitat. These fish do best in packs, but they grow very quickly, and they will address any shortcoming in their diet by attacking each other. To help keep a pack together, you need large tanks (55g+), avoid high traffic areas (hallways) as they startle easily and keep them in cooler water (depress their metabolism and appetite). You will need a steady supply of appropriate foods (ie: feeder rosy barbs) and it's very beneficial if they can be migrated over to non-live foods (ie: Arrowana food sticks, Mysis shrimp, Krill, Beefheart etc). These foods are healthier and dampen their killing instinct (making them better neighbours with each other).
Plecos
Security is a chunk of driftwood that they can attach themselves upside down underneath, in a dark corner, and slowly munch away at the fibre. Non algae-eating plecos may prefer other shelters (clay pots, stacked rockwork, piled wood etc). The armoured plating on most plecos makes them relatively invulnerable to most attacks. The first hard rays (spines) on the dorsal and pectoral fins are reasonably effective weapons for offence and defence, and can be used to hook themelves in the throat of a fish, making them difficult to swallow. Their vulnerabilities are their soft underbelly (which is soft to allow expansion from large meals. Security is achieved with a tight fitting 'parking' spot which prevents predators from being able to flip them over to their soft underside. Beware of putting adult plecos together, as they will either become fast friends or will attack each other.
Tetras & Rasboras
Shoaling fish, they do best in large groups (minimum 5). Suitable tank-mates which would keep them secure are Corys, Kuhlis, Hatchetfish, small barbs (Cherry or Checkered), and other tetras of similar sizes. Habitat should include size appropriate shelter, such as a variety of plants or closely intertwined driftwood. Tetras do not give a tremendous amount of consideration to security, and are a food fish in nature. In an extremely aggressive environment, then will adapt and stay under cover, but in anything resembling a normal environment, they will parade around in a group.
The tightness of their group will sometimes indicate their level of security. Staying in a tight group is more demanding, from effort and loss of feeding opportunities. As soon as they establish that there are no predators, the group size will expand outwards, sometimes loosely (Neons) or to fill the tank with each fish an equal distance from each other (Black tetras). Some have a tendency to stay more active together (Congo and Rummynose tetras), or just stay together more quietly (Penguin tetra). As always, environment plays a large part in how they will choose to behave, so predictability tend to anecdotal.
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