A guide to aggression levels

Opening
This will be a guide to help anyone. Understand what specific variety of aggression a fish has, and how to reduce that aggression if possible. Now, this list includes a personal labeling that I set up a while ago to categorize my fish. The trick to this categorization is that one fish can be in multiple categories, such as bettas. Bettas could easily fall into A0, A2, A6, A8, A10, A11, A11, A12, A14, A15, A16, A17, A18 and A24. Putting aggression into one box is not easy, but understanding the different types is very important.

A0
Intimidated
These are fish that simply just don’t do well with other fish. They may be too small, scared, or just won’t thrive with other fish. Fish like these include clown killifish, as they really really need to reproduce to keep their population up, and their eggs will be eaten if kept with other fish.

A1
Peaceful
These fish pose no threat to other fish. These include creatures such as snails, and fish as big as the flag tailed prochilotus.

A2
Territorial but defenseless
These are fish that may hurt other fish, if provoked and can focus on one specific fish, but will lose any fight if put up against something with the tiniest bit of power. Fish such as bettas and gourami.

A3
Species only
These are fish that simply just need to be kept in their own tank with their own species. This includes most puffers, slow eaters, such as pipefish and sea horses, as well as goldfish and koi.

A4
Territorial before breeding
These are fish that get a bit defensive when trying to find a mate. While they may try to dart at a fish for an instant, it will not last. Fish like these include peacock gudgeons, and Apistogramma

A5
Nippy when out of correct numbers
These are fish that when they’re not kept in their correct number possibly will nip other fish. Fish like these include live bearers, tetras, and barbs.

A6
Aggressive to spawning partner
These are fish that the male will constantly be trying to breed with the female of the group, effectively harassing the female, leading to stress and possible death. This includes fish such as live bearers, and pea puffers.

A7
Nippy, but not fatal
These are fish that may nip at our longer finned friends, but over all can still be kept in somewhat community tanks. These include fish such as tiger barbs and “peaceful” puffers.

A8
Territorial after breeding
These are fish that will defend their eggs from other fish, so they won’t be eaten. These include fish such as Apistogramma, rams, and discus.

A9
May be toxic
These are fish that can be kept fine in community settings, but when are extremely stressed, may release a toxin that will harm other fish. Fish like these include some types of Corydoras.

A10
Territorial
These are fish that are just naturally territorial. This does not mean that these fish can’t be kept together, but special care needs to be taken. Fish like these should not be kept with peaceful fish, as they will hurt them. To keep these fish together, the tank should be overstocked to reduce aggression, using the [African cichlid principle|http://aminoapps.com/p/v2xxqe] to stock the tank, and every fish should be able to defend itself. Fish such as these include African cichlids.

A11
Aggressive after breeding
These are fish that will attack each other after breeding to protect the new born fry and eggs. Fish such as these include bettas.

A12
Aggressive towards own species
These are fish that are specifically aggressive to their own species, rather than other fish, and should be kept as the only fish of this species in the tank. Fish such as these include African butterfly fish, and bettas.

A13
Pairing fish aggression
This refers to fish that when they’re kept in maybe a group of three, where there is one pair, and an extra tag along. This is an extremely stressful environment for the tag along, which usually leads to death. Fish such as these include rams, discus, and angels.

A14
Hierarchy fish
These are fish that set up their own systems of power, to decide which fish is in charge. Typically, some fish don’t get to eat in this environment, so special care needs to be taken to feed these fish. Fish such as these include female bettas, and African cichlids.

A15
Aggressive towards similar species
These are fish that will be aggressive to fish that not only are the same species, but will attack fish with similar appearances to their own species. Good examples of these fish are bettas, and crawfish.

A16
Predatory towards inverts
These are fish that simply view any kind of invert as food, and shouldn’t be kept with other inverts, unless the inverts will become food. Fish like this include every puffer species.

A17
Predatory towards smaller fish
Fish like these are fish that can be kept with fish larger than them, but will eat anything that can fit into their mouths. Fish in this group include blue acara, and Scarlet Badis.

A18
Aggressive with age
These are fish that will become aggressive when they get into old age. They may be fine in other communities for a bit of time, but will become aggressive when they get decent sized and old. Fish like these include algae eaters, and Pictus Catfish.

A19
Toxic
These creatures are fish that will make the water of their tank unsuitable for any other creatures, as it will kill them. Creatures such as these are a lot of amphibians, such as fire bellied toads.

A20
Injected with hormones
These are fish that were injected with hormones to achieve these colors. These fish need a tank to themselves, with no other tank mates. Fish such as this include flowerhorns that are not cherry dragons.

A21
Aggressive
Aggressive fish are simply that, aggressive. They may be able to be kept with other fish with the [African cichlid principle|http://aminoapps.com/p/v2xxqe], but is a major risk. Most fish such as these include just larger new world cichlids.

A22
Predatory
These are fish that will eat pretty much anything other than their own species that’s put into the tank. Fish like these include fahaka puffers, that should either be kept alone, or in massive tanks with other puffers.

A23
Alone from attribute
Fish in these categories just include stuff like electric eels, where it’s not recommended to keep them with any other fish, even their own species due to their ability to just easily kill each other.

A24
Alone from personality
This could be pretty much any fish that just doesn’t do well with any other fish. A lot of bettas can be like this, as well as some personality driven cichlids.

A25
Alone
These are just fish, no matter their personality, should just be left alone in their tank. Fish such as these include squid, octopus, and cuttlefish.

Steps to reduce possible aggression
If your fish is overly aggressive, there are a few options to just keep aggression down. Obviously, do not create a predator-prey relationship between the fish you want to keep, but some other fish can be managed. The easiest strategy is to not feed fish live food. While it is generally healthier for them live food, it will lead the fish in the tank to think in a way that makes fish around them look like food. Other options are to raise the fish in a community tank while they are younger, so they’ll become accustomed to their surroundings. The [African cichlid principle|http://aminoapps.com/p/v2xxqe] can be applied to relieve some aggression. It is also always a good idea to add the most aggressive fish into a tank last. This means that fish will set up its territory with the other fish in mind, and not view them as intruders.

Conclusion
Now you know the ins and outs of fish aggression. It’s important for all of these to be considered when building a tank for the specific fish you want. Of course, tank mates also need to match water requirements, but it’s always good to label a fish, to have a deeper understanding of how to keep it.