Some fish species in the aquarium hobby can be very aggressive but reward the keeper with fascinating behaviours if kept in an aquarium that meets their needs. They can come from all over the world and in all manner of shapes and colours. Common species include snakeheads, piranhas and large cichlids. As these fish require more specialised care, keeping them in the home aquarium can require more commitment than other species. This should be considered before purchasing any of these species.
Water requirements
Water chemistry requirements for aggressive fish species will vary as they originate from many different habitats. It is important to check with your OATA retailer for any species-specific requirements before purchasing. Regardless of species kept, it is essential that the following parameters are followed:
Ammonia: Zero mg per litre
Nitrite: Zero mg per litre
Nitrate: Not to exceed 20 mg per litre above normal tap water levels
Biology
Most aggressive species will grow to a relatively large size, with many exceeding 30cm. However, there are some species which remain much smaller, such as the Bucktooth tetra (Exodon paradoxus), which will grow to approximately 10cm.
Many aggressive species, such as wolf fish or snakeheads, will be aggressive to any other individual fish.
Some species such as piranhas or Bucktooth tetra are social fish and will need to be kept in shoals. Piranhas will need to be kept in groups of at least five, but bucktooth tetra should be kept in numbers of at least 12 to reduce the chances of one fish being bullied. Fish kept singly or in groups that are too small will often become stressed. Although these species require groups of their own, they will struggle to mix with other species.
Due to their predatory habits, some fish will have very sharp teeth and should be handled carefully. Please see our Hazardous Aquatic Animals guidance for more information.
All these fish can thrive for many years in a healthy aquarium with good water quality.
Some snakehead species are invasive species and banned from sale in the UK. If in doubt, check Introduce or keep non-native fish and shellfish for a list of species that can be kept for ornamental purposes and ask your OATA retailer for guidance.
Aquarium requirements
As a general rule, you should within reason, buy an aquarium as large as possible. Ideally, it should be able to accommodate at least a small shoal of piranhas or bucktooth tetra or be large enough to safely house any other aggressive species. Ask your OATA retailer for recommended tank sizes for the aggressive species you are interested in. The larger the aquarium, the more stable the environmental conditions such as temperature and water quality will be. Some sedentary species, need a large volume to be able to control water quality, but may not need quite the same volume as active swimmers. Whatever the size, a filter is always essential.
Usually, the tank should also have aquarium gravel or sand and ideally live plants present to provide cover. If live plants are not used, then the addition of plastic plants and suitable ornaments is recommended, such as plant pots or model caves to provide the fish with shelter. Some larger species have a tendency to disturb aquarium décor and can sometimes knock over aquarium ornaments and plants. Any ornaments, including rocks placed in the aquarium should be securely positioned to avoid them from becoming dislodged and damaging your aquarium. Aquarium heaters should also be protected from damage. Care should be used when choosing aquarium gravel or sand to ensure that any bottom-dwelling fish aren’t injured by sharp edges and that it can be kept clean enough so that it doesn’t impact on water quality.
A heater is required to maintain a suitable temperature all year round. To minimise fluctuations in water temperature, the aquarium should not be situated near any draughts or heat sources. It should also be out of direct sunlight and away from loud noises, vibrations and sudden movements. Overhead tank lighting is recommended to maintain a correct day-night cycle.
Water testing kits are essential so that water quality can be checked on a regular basis (once a week) to ensure it does not slip below the water requirements stated above.
Introducing your fish
Before adding any fish, seek advice from your OATA retailer to make sure that your aquarium is an appropriate size for the species you would like to keep. Check that the water quality in your aquarium is suitable i.e. levels of ammonia and nitrite are zero. Only increase the number of fish you have in your aquarium slowly as the population of beneficial bacteria established when maturing your aquarium filter need to increase every time more fish are added and feeding increases. Overstocking or stocking your aquarium too quickly can result in ‘new tank syndrome’. This occurs when there are not enough nitrifying bacteria to cope with the increased waste from the fish, leading to unhealthy levels of ammonia and nitrite which may cause fish to become ill or die.
Healthy fish have clear bright eyes, undamaged fins, intact scales, no ulcerations or bumps, appropriate swimming behaviour and steady breathing. Do not purchase a seemingly healthy fish if sickly fish are present in the tank with it. Signs of disease can include clamped fins, flicking against gravel or décor and shimmying (shaking). Diseases can be easily carried by fish that do not show any clinical signs. If in doubt, ask your OATA retailer for advice as they will have in-depth knowledge and experience.
Your OATA retailer will usually sell your fish to you in a plastic bag. Some aggressive species have sharp teeth or spines and so should be double bagged to prevent any leaks if they break the bag. Some larger species may be sold in boxes or buckets. No matter how your fish is packed, try not to keep them in it for too long. Once purchased, take your new fish home as quickly as possible because fish are easily stressed by bright lights, extreme temperatures, noise and movement.
Once home, your fish will need to acclimatise to their new environment and a common method of doing this is known as the ‘floating bag’ method. Switch off the aquarium lights and take the bag containing your new fish out of its outer wrappings carefully, avoiding exposure to bright light. Float the bag in the water of your tank to ensure the temperature in the bag is the same as the aquarium water. After 10 minutes, slowly introduce small amounts of aquarium water into the bag containing the fish for up to 20 minutes. Once complete, carefully release the fish into the aquarium whilst introducing as little bag water into the aquarium as possible. After this, dispose of the bag and any excess water appropriately. Monitor your new fish carefully for the first week, paying particular attention to water quality. If in doubt, contact your OATA retailer for advice.
Some larger aggressive species may be sold in a bucket or box and so drip acclimation might be a better method to use. This could be achieved by keeping the fish in the container it is sold in and a small airline siphon started to drip water into the container, slowly changing the water parameters to that of the aquarium. Ensure that the temperature does not fall too low during this procedure. Once conditions match, net the fish from the container and place into the aquarium. Dispose of the water in the transport container appropriately.
Monitor your new fish carefully for the first week, paying particular attention to the water quality. If in doubt, contact your OATA retailer for advice.
Maintenance
At least once every week, a partial water change of 25% is strongly recommended (a siphon device is useful to remove solid waste from the gravel). The water should be tested regularly (at least once per week) to ensure that ammonia and nitrites don’t build up. Ensure that the replacement water is treated with tap water conditioner to remove any harmful chlorine or chloramine present before adding to the aquarium.
Filters should be checked for blockages. If the filter needs cleaning, do not run it under the tap because any chlorine or chloramine present may kill the beneficial bacterial population that has established in the media. Instead, it should be rinsed lightly in the tank water which is removed during a partial water change as this reduces the amount of bacteria which are lost.
Good husbandry is essential as these fish can be stressed by even the smallest amounts of ammonia and nitrite which may then cause them to develop various diseases. Test the water to monitor the ammonia, nitrite and nitrate levels, together with pH and water hardness every week, especially during initial set-up and after adding extra fish.
What to watch out for
All fish will have slight variations in their behaviour or appearance, but keeping an eye on any changes in the following will help to identify any potential problems before they become a real health issue:
- swimming behaviour – hanging at the surface, sitting on the bottom or erratic swimming
- colour – turning a darker or paler colour than normal
- temperament – changes in level of aggression or hiding more than normal
- breathing – gill covers moving at a slower or faster rate than normal
- appearance – development of white spots or fluffy growths, loss of fins or scales
- condition – increase or decrease in body weight and condition
- feeding – reduced intake or lack of interest in food
If you are concerned about the health of any of your livestock, then test your water quality and contact your OATA retailer for further guidance.
Feeding
Aggressive species are likely to be carnivorous and will need various foods depending on their natural diets. They will all need to be fed the appropriately-sized food as they will often ignore most of the foods used to feed smaller fish. Many manufacturers make large pellets designed for larger individuals. Some species will not take prepared feeds and will require larger frozen food items such as frozen mussel, shrimp or lance fish, or if they are struggling to feed, they should be fed on large insects. Live fish should NOT be used to fed other fish in the home aquarium.
As these fish consume large meals in one sitting, aggressive species should not be fed more than once a day. Take care not to overfeed as this can lead to a build-up of uneaten food which breaks down releasing toxic waste into the water. If in doubt, ask your OATA retailer for advice on appropriate feeding levels.
Compatibility
Due to their aggressive nature, these fish should not be mixed with any other species (unless they require company of their own species as stated above).
Breeding
Due to their aggressive nature and potentially large size, it is unlikely these species will breed in a home aquarium. Some aggressive cichlid species may breed as described in our ‘How to care for American Cichlid’ care sheets.
Checklist
Before purchase make sure:
- You have the appropriate equipment and position for the aquarium.
- You have researched all the species in which you are interested and your final choices are all compatible.
- You are familiar with how to transport and release your fish.
- You are aware of the daily, weekly and monthly maintenance your aquarium will require.
- You are prepared to look after your fish properly for the duration of their life.
Shopping list
- Glass or acrylic aquarium
- Filter*
- Heater
- Lighting (required for live plants)*
- Gravel or sand
- Tap water conditioner/dechlorinator
- Water testing kits (ideally ammonia, nitrite, nitrate, pH and water hardness)
- Gravel cleaner/siphon cleaning device (recommended)
- Aquarium decorations and/or live plants
- Bucket for water changes
*may be included in branded aquarium sets but can be purchased separately.
Before purchase make sure:
- The aquarium is of a suitable size that ideally can accommodate the fish once they are fully grown.
- Water parameters are as advised in this leaflet.
- Aquarium is cycled and ready to receive fish.
Top tips
Always buy…
test kits and regularly check the water for ammonia, nitrite, nitrate and pH. This will allow you to make sure the water in your aquarium is not causing welfare problems for your fish.
Establish a routine…
for testing the water in your aquarium. Record your results to enable you to identify fluctuations quickly. Also check the temperature of the water.
Maintain…
the water in the aquarium within the accepted parameters highlighted above. You may need to do regular water changes to achieve this.
Always wash your hands…
making sure to rinse off all soap residues, before putting them into your aquarium, or use long sleeved rubber gloves. Wash your hands again afterwards and certainly before eating, drinking or smoking.
Never siphon by mouth…
A fish tank can harbour bacteria which can be harmful if swallowed. Buy a specially designed aquarium gravel cleaner which can be started or primed without the need to place the siphon in your mouth.
Five Welfare Needs Checklist:
The Animal Welfare Act 2006 states that all pet owners have a legal duty of care to their pets. Anyone who is cruel to an animal or is found not to be providing the five animal welfare needs, as listed below, can be prosecuted.
- A suitable environment e.g. appropriately sized tank (with water heater if tropical set up) within a suitable location in your home.
- A suitable diet which meets the needs of your chosen fish.
- Behaviour – Fish are able to exhibit their normal behaviour e.g. hiding places for timid fish, enough room for fish to swim freely.
- Companionship – Ensure you know whether your chosen fish need to be kept with, or apart from, other fish.
- Health – Protected from pain, injury, suffering & disease e.g. you are aware of the daily, weekly and monthly maintenance that your aquarium will need.
Remember...
Water quality test kits are a necessity not an optional extra.
You must be prepared to look after your fish properly for the duration of their life and provide an aquarium which can accommodate your fish when fully grown.
Never release your aquarium animals or plants into the wild. It is illegal and for most fish species this will lead to an untimely and possibly lingering death. Any animals or plants that do survive might be harmful to our native countryside. Take care to properly dispose of any soiled substrate (e.g. sand or gravel) water or decorations so that non-native organisms do not enter natural watercourses.
