How Important Is Fish Compatibility ?

How Important Is Fish Compatibility

For building a peaceful community tank, the compatibility of fish is essential. Often, out of excitement, we end up stocking fish in our tanks that are not compatible with each other and to our tank’s conditions. It is a blunder that beginners could make. Therefore, choosing fish that get along with each other is important to build a community tank where fish are happy, stress-free and healthy.

Understanding fish compatibility 

Fish compatibility is the ability of fish to coexist peacefully. While some fish species have a mild temperament and can be kept with other species, aggressive fish act better when kept alone. Therefore, when you select fish for your tank, check their behavioural pattern and know whether they are compatible with each other or not. The right combination of fish in the stock is the key to a healthy and peaceful community tank.

Now, the question is how to evaluate fish compatibility and on what factors it is determined. Fish compatibility can be discussed under three sections which are basically based on their temperament.

  1. Peaceful or non-aggressive fish: Most of the tropical fish fall under this category. They are called peaceful or community fish species. These fish usually have a mild temperament and do not mess with other fish species of the tank. So, they can be kept with any other fish. Some of the best examples of non-aggressive fish include swordtails, guppies, most of the tetras, mollies, platies and danios.

If we look at the features of this category of fish, they are generally small in size and can be kept in shoals with their own species. Make sure you keep them in groups of 4 to 6 fish for the best behaviour. It is because some of the fish in this category perform well when kept in groups but get aggressive and feel lonely when kept singly or in pairs.

  1. Semi-aggressive fish: The fish that fall under this category are sharks, eels, barbs, gouramis and loaches. They are generally peaceful but may show aggressive behaviour in specific situations. Since, in most cases, males are more aggressive than females, it is recommended to have only one male with other species at a time.

At the same time, there should be plenty of hiding places in the tank where the peaceful fish species can retreat from aggressive tank mates and hide. If you have a large tank, you can keep multiple semi-aggressive fish together only if there are plenty of hiding spots like decorations and aquarium plants to break up sightlines. Semi-aggressive fish establish their own territory on the part of the tank. Therefore, if the tank is large enough, and each fish can create one, there will be no problem.

  1. Very aggressive fish: This is the final category of fish based on their temperament and includes cichlids like African cichlids, Oscar cichlids, large catfish, and plecostomus. Most of the males of these species show territorial behavior when kept together. Therefore, it is best to keep them alone or in tanks where other fish are of their own species.

Since maintaining peace gets very difficult in aquariums that have aggressive and semi-aggressive fish, they are not recommended for new aquarists or beginners. Such fish often fight with other species for no reason and keep nudging them. It makes the other fish come under stress that affects their health and well-being. Sometimes fights between fish end up getting either the smaller or weaker fish injured or die.

What are the danger signs?

Now the question is how to know that your fish is under stress and it is because of some other fish species in the tank. Well, the behavioral pattern of a particular fish can easily tell whether it is happy or not. Some clear signs that can indicate stress or bullying include the following-

  • Fish is not eating properly as the other fish is pushing it out of the way and not giving space.
  • Fish is constantly hiding under aquarium plants, filters or décor items you have in the tank.
  • You can see some injuries on the fish’s body, like missing chunks from the tail.
  • Fish is swimming in a strange manner as it has some injury on the fins or tail
  • Fish is chasing the other fish

The fish also show some of these particular signs while mating. Therefore, you have to check your fish daily to see if they are showing some strange behavior. If you think a particular fish is getting bullied by the other one, it is best to separate them in different tanks.

How to stock the tank?

Before you buy the fish for your tank, you should-

  • Research about the fish’s behaviour– This is the foremost thing you must do before getting fish for your tank. You can find enough information online, or you can ask fellow aquarists to know the behaviour of fish. Check out the temperament of the fish- peaceful or aggressive before stocking
  • Do not overcrowd the tank: If you have a small tank, and you are stocking it with plenty of fish, it could create a problem even if all the fish are peaceful. It is essential that each fish gets enough space to swim freely and move anywhere in the tank. Overcrowding causes aggression and stress among the tank mates. Also, the fish will be unable to live up to their full potential in both size and colouration. At the same time, do not forget that each fish is territorial in varying degrees.

Apart from these two important points, there are a few things that need consideration regarding your tank. You have to make sure that your fish are compatible with your tank as well. Here are a few things you should keep in mind that can help determine fish compatibility.

  • Size of your tank: The more space fish have, the better they tend to get along with each other. Overcrowded tank often leads to agitated and stressed fish. The thumb rule is to accommodate one inch of adult size fish per net gallon of aquarium capacity. However, this doesn’t hold true for territorial fish that need more space. Do not forget that what we think is a big tank is just a fraction of space fish usually have in their natural habitat.


  • Dimensions of your tank: It is always a good idea to invest in a tank that is wider in dimensions. It gives fish enough space to swim and spread out that, in turn, helps in getting along with other tank mates better. If you have a tall and narrow aquarium that looks appealing and easily fit into narrow spaces, the fish won’t get the desired space to swim. It gets more difficult for the territory fish.


  • Aquarium plants and decorations: They work best for fish that are territorial as they provide physical boundaries to them. While the driftwood and rocks help define territories for fish like cichlids, bushy aquarium plants provide hiding places and great habitats for fish like barbs and tetras. This way, fish mind their own business and coexist peacefully.


  • Age and size of the fish: If you are buying different species of fish (some peaceful and some aggressive) in their juvenile phase and stocking in the tank at the same time, they may end up living peacefully even when they mature. When it comes to size, make sure all the fish in the tank are of almost similar size. It is because fish are opportunistic, and the bigger ones will never leave a chance to swallow the smaller ones.


  • Gender of the fish: Generally, it is seen that male fish are more aggressive and exhibit territorial behaviour, especially when mating, for example, cichlids. Therefore, it is better to avoid having more than one male cichlid in the tank, particularly when female cichlids are there. Likewise, make sure keeping the ratio livebearers of male and female to be 1:3 to keep the mating behaviour of males under control.


  • Water parameters: Once you have checked everything about the fish behaviour and tank’s conditions, you have to ponder upon the water parameters too. Until the water parameters are not within the suitable range, the fish will keep on troubling each other due to stress and shock. Make sure every fish falls under the same parameters requirement like temperature, pH, water hardness, etc. if one fish requires 25 degrees Celsius temperature and the other 15 degrees Celsius, it is better to keep them separate.

Final words: This is just an outline of some of the key considerations while stocking the fish tank. Detailed and thorough research is always key and critical that every aquarist should make before making a move. The three factors that need more attention by any aquarist are- getting risk-free combinations of fish, having the right size and dimensions of the tank and keeping the water parameters within the range. If you are following these three, there is no chance your fish will get into stress and fall sick. Make the right choice and enjoy our hobby.

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