People can keep fish as they wish; however, a conscientious aquarist strives for habitat that is best for the security and comfort all of the species. And anyone who understands the reason for shoaling would never, ever keep one or two:
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.
The primary advantage of forming a tightly knit shoal is to minimise the chances of being picked off by predators. 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. *From Thinkfish
Knowing the above, it is more than apparent that, as Rainbo noted, shoalers stay alive if kept alone but it is less than acceptable and unnatural and leaves them more prone to stress and disease. Maybe I am odd, but I like to measure longevity in years and not months. One of my 10-member Habrosus Cory shoals puttered around on the bottom of their tank for 5+ years. And they shoaled even though they were in a tank with just other Nanos and no predators...as did the Neons, Embers, Chili Rasbora, etc. So to imply they do not need a shoal in a predator-less aquarium is terribly inaccurate.
So, to the OP and others who may wonder: Do what those who have studied fish for years suggest and keep proper shoals so you can enjoy your fishy friends for a long time.
If your dog thinks you are the greatest do not seek a second opinion
Last edited by RussellTheShihTzu; 11-14-2018 at 11:42 PM.
Reason: Removed quote from deleted post.