Deciding on fish numbers

Most aquarium fish can be classed into one of three living preferences:

Schooling and shoaling:
These fish, depending on the species, live in groups that range form a few hundred to a few million individuals. Home aquariums are most often not able to hold groups that large, but the bigger the group the better it is. I usually recommend that one should aim to keep 10 – 15+ individuals per schooling species as there is no excuse to not do so if stocking a new aquarium. Unfortunately, some people find out that they have only a few individuals from a schooling species after the aquarium is fully stocked, in which case it is best to try and increase the numbers to at least 6 individuals per species or to find them a new home. In some way, fish are aware of individuals up to a point, at which the individuals become “many”. I think that 6 individuals is this point for many species. One of the most important functions of schooling is to protect the individual fish from predators, either by letting the weaker fish in the group be picked off first (as easier prey) or appearing as one larger fish. The main difference between schooling and shoaling fish is that shoaling fish will normally only swim in a tight formation when threatened, usually going about their own business (for example, Trigonostigma heteromorpha). On the other hand, schooling fish (such as Paracheirodon innesi) will spend most of their time swimming close together, even to the point of facing the same way. The group includes fish like tetras, rasboras, danios, barbs, many loaches and rainbows. One unusual member of this group is Neolamprologus brichardi, a shoaling cichlid.
Small groups:
There are a few different variations of small groups which can be found. These include small groups of social fish, which do not have much social structure (such as livebearers) or which have a specific social structure (for example, cichlids); closely knit family groups; pairs of breeding male and female couples; harem groups of one male and a number of females (often seen in many Apistogramma species), or quite rarely, one female with some males. As with schooling fish, small groups provide security for individuals. For some mildly aggressive species, such as Pterophyllum scalare, it may even be possible to keep them peacefully only individually, in proven breeding pairs or in small groups of more than 6 individuals because the dominant fish can then spread the aggression over multiple individuals, instead concentrate it on a single one.
Some of these fish are too aggressive to keep with any others of their own kind, and in some cases, even with other fish which would occupy the same area inside the aquarium, while others simply do not interact with one another on a regular basis. This group includes some loaches, cichlids and gouramis.

It is quite important to try and keep the fish in appropriately sized groups as some may otherwise display odd or aggressive behaviour. The easiest way to find out appropriate stocking numbers is to research the conditions in which the species is found in the wild.

Some basic research showed me that Danio margaritatus and Yunnanilus sp. ‘rosy’ is a peaceful, mid-water schooling fish, which automatically means that I should be considering 10 individuals per species. This is a good number to start with, and there is always the option of adding more later.

Pseudosphromenus dayi, on the other hand, is a solitary fish which breeds in pairs. Males may occasionally be persistent, so I decided that it is better to have 1 male and 2 females, to give the females a bit of a break in case of uninvited attention.

So for my first “final stocking”, I will be aiming at the following:

  • 10 × D. margaritatus
  • 3 (1m 2f) × P. dayi
  • 10 × Y. sp. ‘rosy’

It is very common for final stock to evolve with time, which is why I am referring to this as my first one. As for how I decided on the total number of fish? That is rather difficult to explain as there are no set rules, nor have I seen any good guidelines. I chose the number based on my experience and I always base my decisions on adult size. The stocking numbers are also affected by the amount of plants in the aquarium as they will use up ammonium. For an aquarium which is 60 × 30 × 30 cm in size, I would normally expect to stock between 6 individuals of the larger species I list and 25 individuals of the smaller species. I would also stock conservatively if I pick female livebearers because they will drop fry and it is best to reduce chances of overstocking.

The best advice I can offer on stocking is to not stock more than one feels comfortable with, even if others say that the aquarium will take more fish, and if one is being advised to stock less than one plans to, to try the lower stock first.

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