Fish Types and Categories

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Understanding the main types of fish available will help you to plan your aquarium, choose your fish, and speak the language of a fish-keeper

Four different fish

Fish are similar to people in more ways than is first apparent. Fish all have distinct personalities and behaviours that may apply to a group of fish, a species, or even just an individual. With time, observant fish-keepers will learn to recognise traits in their fish and know how that fish will react to certain events. It is this level of understanding and communication between the fish and the fish-keepers which makes the hobby so addictive and rewarding

Most fish exhibit behaviours that are common within their group or species although it is not uncommon to have a 'rogue fish' that behaves unusually.

Shoaling fish

Many common aquarium fish, particularly tetras and barbs, are shoaling fish. A shoal is a group of fish that will stay near each other and prefer to live in a large group, rather than individually. Smaller fish often shoal in the wild for protection and to confuse predators; it is harder to keep an eye on one fish when there are many more zooming around!

In the aquarium shoaling fish should always be kept in groups or they may become shy, timid and stressed, leaving them open to disease and bad health. In groups, shoaling fish are healthier and exhibit improved colours. A shoal should consist of around 6 or more fishes.

Territorial / Aggressive Fish

Aggression in fish is usually related to defending territories in the aquarium. The purpose of defending a territory may be to establish control over food sources, protect young fish, establish breeding sites, display superiority to attract mates, or a number of other reasons. These reasons are all important in the wild where intruding fish can be simply chased away from a territory. In the aquarium however, fish cannot be simply chased away, this is when problems occur.

Between territorial fish in the aquarium, one fish is usually the stronger, dominant fish. A dominant fish will continue to chase intruders away, even if there is nowhere for them to go. Continual aggression like this will weaken the other fish, causing damage and disease, in some cases, even death.

Some territorial fish are only aggressive to other fish of the same type, in which case just one fish of that type may be kept. Other territorial and aggressive fish can be kept together but it would be wise do a little research before attempting this. A good example is the rift lake cichlids, which are too aggressive and territorial to be kept with most peaceful fishes. These fish can be kept with other rift lake cichlids in a highly stocked aquarium where territories are hard to establish, and so aggression is divided and reduced.

Predatory fish

Fish species feed on a wide range of natural food sources, some are primarily vegetarian (herbivore's), some feed on meat and vegetation (Omnivore's) and some primarily on meat and animals (Carnivore's). Predatory fish are designed to hunt and eat live animals, most often-other fish, and will do the same in the aquarium. Predatory fish may not always be large in size but may be capable of eating other fish up to half their size quite easily.

Ensuring that predatory fish are not kept with smaller fish should be a rule that is applied loosely to all fishes. If there are other fish in the aquarium that will easily fit into the mouths of larger fish then the chances are they will be eaten, regardless of the larger fishes natural feeding habits. Predatory fish should therefore only be kept with fish of a similar or larger size.

Solitary fish

The opposite of shoaling fish, some fish are solitary and prefer to be on their own. Solitary fish may just prefer to live with fish of a different species to their own; others will prefer no other fish in the tank. Many solitary fish can be kept quite happily in a community aquarium providing they are provided with suitable hiding spots.

Soft water / Hard water fish

In fish-keeping you may often hear terms such as hard water, soft water, pH level, acidity and alkalinity. Different species of fish prefer different levels of these water 'qualities' depending on where they come from in the wild. The difference between soft and hard water is, in very simple terms, the amount of mineral salts in the water.

Water with a high amount of mineral salts such as calcium or magnesium may be very hard whereas water with little mineral salts (rainwater or distilled water) may be soft.

pH is closely linked to hardness but is a measure of how acid or alkaline the water is. The pH scale runs from 1-14, 7 is neutral, above 7 is alkaline and anything below 7 is acidic.

Most fish live happily between pH 6.5-7.5 but some will prefer more alkaline or acidic conditions. It is possible to alter your waters hardness and pH through the use of chemical treatments, mechanical treatments (equipment) or by using other water sources. To alter your waters property it is advisable to seek advice either from books or your retailer.

Before purchasing any fish it is wise to test your water to find out what kind of conditions are present, you can then choose fishes based on those conditions. Aquarium test kits are easy to use and widely available at most retailers, many retailers will also carry out tests for a small fee.

Brackish fish

Some fish come from habitats between the ocean and the river where the water is neither freshwater nor saltwater but brackish; a mixture of the two. Brackish fish are highly adaptable to different conditions so do not need the additional equipment of a marine aquarium and are as easy to keep as freshwater fish.

Brackish fish should have around a quarter to half the amount of salt recommended for marine aquaria. Make sure you check the salinity (salt level) of the water the fish come from (the retailers tanks) before they go into your brackish tank, as a sudden change can be harmful. Remember that the majority of freshwater fish will not tolerate high salt levels so cannot be kept in brackish aquariums.

Marine fish

Marine fish come from the oceans and reefs and so need to be kept in water with a high salt content. This means they cannot be kept with normal freshwater fish. Because the oceans are so vast in size, water conditions are very stable in comparison to freshwater rivers and lakes where the quality of water fluctuates throughout the year. Due to this high level of stability, marine fish require a much more constant level of water quality in an aquarium and hence, a higher level of care. To keep marine fish you should have a reasonable amount of fish-keeping experience, be willing to undertake further reading and to pay for additional equipment.

Algae eating fish

Many catfish and loach species are natural algae eaters. These fish can be very useful in an aquarium, keeping algae down to a minimum. Care should be chosen when choosing these species however as some, such as the popular plecostomus, can get quite large. Small algae eaters are ideal for planted aquariums where they will remove algae from glass, decor and plant leaves, normally without damaging the plants.

Algae eating fish will normally only stick to types of algae that suit their diet; few will eat hair or brush algae, a common occurrence in aquaria. Some algae eating catfish, such as Ancistrus sp., require some bog-wood in the aquarium. These fish will use the wood both for hiding spots and to graze upon. The lignin found in wood is thought to be a vital part of the fishes diet. Algae eating fish must be fed additional food in the form of specially designed sinking wafers, tablets, or granules to supplement their natural diet.


Other catfish and loach species are scavengers and will hunt around the aquarium floor looking for waste food material. These fish often use sensitive barbels to locate food. It is important that the aquarium substrate must be smooth and rounded to avoid damage to these sensitive barbels. Again, these fishes must be fed additional food to supplement any they may find. Good 'natural' foods to feed both algae eaters and scavengers include lettuce, cucumber and shelled peas.

Surface dwelling fish

Some popular fish species are natural surface dwelling fish. Many of these will also spend time in other areas of the aquarium but those that stick to the surface should be given special care. Surface dwellers such as hatchetfish and some gouramies are quite timid fish and may be stressed by the bright, open surface environment of an aquarium. These fish should be given plenty of hiding spots by introducing floating plants at the surface.

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