Many fish shoal or form schools in nature, and the reasons why they do this have effects that can be seen in the aquarium. The difference between a school and a shoal is subtle; a school of fish is a tightly regimented formation where fish swim an equal distance apart and the group turns as a whole. A shoal is a less formal arrangement where fish swim close together and follow the same general direction but each fish may do as it pleases. Virtually all grouping freshwater fish found in the hobby are shoaling fish rather than schooling and should be referred to as such. Natural shoals can range from groups of tens to hundreds (Such as Corydoras), hundreds to thousands (such as Neon's & Piranha's) and in the case of some open water marine fish, in millions. Knowing why your fish shoal will help you to understand their behaviour in the aquarium and in some cases, help to eliminate potential bullies and compatibility problems.
Avoiding predatorsThe primary advantage of forming a tightly knit shoal is to minimise the chances of being picked off by predators. If you are a small fish, the chances are you are also a desired food item for larger fish, and sooner or later someone is going to try and eat you. A single small fish would have to face a one on one battle for survival if a predator should arrive but as a group, not only is the chance of you being eaten heavily reduced (hopefully it will be someone else), but the chances of the predator catching anything at all is much reduced. In a shoal, fish can use the intelligent solution of a co-ordinated defence to avoid and confuse predators.
The first step is to try and not look like a small fish and this can be done by staying as close to your fellow fish as possible. If everyone in the group does the same thing, a tightly knit ball of fish is formed which from a distance, appears as one large object, hopefully far to large too be eaten. The predator, seeing such a large moving object, is more likely to not only ignore it as it appears too large to eat, but may even actually avoid it thinking that the strange object could be a threat.
If this approach doesn't work and the predator attempts an attack, then the art of confusion can be employed. As a predator charges into a shoal, the fish split into two directions around the predator and re-form around the back, leaving an empty space
around the predator as it swims through the group. From the predator's point of view, it is impossible to pick out one individual fish amongst the group as they scatter. Without a fixed target, the predator is simply hoping that a fish will happen to swim into its gulping reach, and has a much-reduced chance of catching anything. Whilst this game is played, the shoal will head for a hiding spot and if they all make it there in time the predator is left with nothing and will eventually swim away looking for an easier target. 
FoodMost small fish are opportunistic feeders and will eat whatever is available; normally food items such as insects, water-born organisms or substrate dwelling organisms are preferred foods. These kinds of foods will be found in concentrated areas in the wild, rather than being evenly distributed, and will need to be found. As a large group, there are many more eyes and other senses to search for such foods and if, as an individual, you cannot find much, someone else in the group probably will. As soon as one fish finds food and leaves to group to feed, the others will follow, also gaining access to the concentrated area of food.
Hierarchies
Many tropical freshwater fish exhibit a hierarchy structure where there is an order of dominance within the group. In this situation, the benefits of a shoal are available to all the fish but the strongest fish have first access to food and, showing dominant colours, are more attractive to females. In the animal kingdom, a hierarchy ensures that the majority of genes passed on to the next generation are those of the strongest fish, thereby helping the species remain strong. Amongst a group of fish, the hierarchy is established by normally relatively peaceful means. Displays of strength are commonly seen where two fish will swim alongside each other and rapidly beat their fins, especially the tail - the fish that produces the strongest current is the winner and assumes a higher position. This method is quite peaceful and causes no harm to either fish but in certain circumstances, damage can be done. 
If hierarchy fish are kept in small numbers in the aquarium, the dominant fish, in order to maintain its position, will continually confront the other fish. If there are only two or three other fish of that species, then they will continually be chased, confronted and battled with, and will be denied feeding rights by the dominant fish. This will result in the less dominant fish becoming reclusive and not feeding, leading to ill health. In fishkeeping, this effect can be seen with many gouramies, swordtails, and even some tetras. Normally it is the males which control the hierarchy so with fish like these it is best to keep only females unless you have a group of six or more, in which case any ratio can be kept. In large numbers the dominant fish has too many other fish to subdue and so only an occasional chase or battle breaks out - just enough to let everyone know who the boss is. Tiger barbs, which are well known for their habit of being nippy, can be controlled in this way. In small numbers they are very nippy towards other fish, but this is just a behaviour caused by an extension of their natural hierarchy reinforcing nature. As a large group of ten or more, tiger barbs will be able to act naturally as a shoal and will not bother with other fish, becoming much more peaceful and far less likely to fin-nip.Shoaling fish that stop shoaling
As we have seen, shoaling has distinct advantages, especially for smaller fish but those advantages only apply to wild situations. In the aquarium environment food is easy to get and its appearance is predictable, predators (should) never occur, and hierarchies generally remain stable. The consequence of such a safe environment is that in many fish, the natural shoaling behaviour is significantly reduced. The effect can be seen most often in smaller fish such as tetras, which mainly shoal to avoid predators. In the aquarium, providing an individual neon, for example, knows that there are other neon's around; it feels quite safe to go off and do its own thing. If an unwanted situation should occur, the neon knows that there are other neon's around and that they will all head together to form a shoal. Because it is only the knowledge that other fish are within sight that allows the fish to act on its own, it is important to still keep shoaling fish in groups, even if they appear to completely ignore each other. A neon on its own will become stressed and probably ill because it has no 'safety net' of a group should trouble occur, whereas a group of neon's will probably eventually disperse and appear to not even notice each other!
Young and oldVirtually all fish will shoal when they are young and this is an extension of behaviour as fry, when the advantages of sticking together to find food and avoid being eaten are greatest. Fish that grow large, have few predators, or live in environments where they hide or have other defences have little need to shoal and being in a group could even be a disadvantage. For these fish the shoaling behaviour as fry or young fish stops when they reach maturity. At this point these fish will prefer their own space and will see other fish of the same type as a threat, either to their own food sources or to their young if they breed. This situation is when territorial and aggressive behaviour develops, usually seen in fish such as many larger cichlids, red tailed black sharks, and some larger catfish. It is the change between young and old fish which explains the reason why you may see many fish of this type in an aquarium shop in the same tank getting along together - they are still young and being in a group is beneficial. If you were to purchase a few fish of this type they would probably get along fine in your tank for a few months but at some point, their will be fights, followed by the dominant fish aggressively bullying the weaker fish, and even other fish in the tank.
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Article - Creating A Community
Article - Community Conflicts
Article - Fish Types
All article text and pictures are copyright Think Fish and may not be used without permission.
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