Update Your Site & Language

Type of Retailer

  • Premium Dealer Premium Dealer

    An officially recognized retailer offering a wide array of Fluval products and expertise.

  • Authorized Retailer Authorized Retailer

    An officially recognized retailer offering a selection of Fluval products.

  • International Distributor International Distributor

    The authorized distributor of Fluval products within a specified country or geographic region.

Items Added to Your Cart

0 Item in Cart

Shipping & Taxes calculated at checkout

View cartCheckout

Fish, Shrimp & Corals


December 13, 2018 | By Tom Sarac

In nature, fish have all the space they need. Aggressive species tend to be separated by greater distances, sometimes meeting only to breed – just like polar bears in the North. Aquariums obviously limit the amount of space any individual fish can occupy and therefore, without precautions and planning, this can have serious consequences – most often resulting in the death of the weaker species.

The best way to avoid such a problem is to carefully research the fish you are planning to keep, and to make sure enough space is provided for all the cohabitating fish.

The following factors are some of the limitations of fish aggression, and I’d like to propose some solutions, such as creating natural barriers, which will greatly reduce the chances for aggressive acts.


Before taking any fish home, carefully research how large your fish will get, and how their behaviour will change when they become an adult. Fish sold in the aquarium trade are usually not adults, and may not yet be sexually mature. With age, the fish will not only grow, but it will also mature and thus demand more space. Make sure that your aquarium is large enough to house your new fish as they grow into adult size.

There is really no general rule that will let you mathematically calculate how big an aquarium should be for any number of fish of any size. However, I recommend you purchase an aquarium as large as possible, and research every species you will stock within it – be it at your local fish store, in many great books that cover this topic, or on the Internet. YouTube can be a great tool to watch your chosen fish species in action as an adult. If someone has bred your fish and has posted a video, then the tank featured is likely large enough.


Find out how many fish of your desired species live together in nature. Fish that live in schools or small flocks in the wild usually have little or no aggression in the aquarium. For some species, it’s important to live in a school, and they may become aggressive or shy or simply do not do well if kept in a group that is too small. Many of the Lake Malawi and Lake Tanganyika cichlids, for example, need to be in a larger group in order to thrive. Kept alone or in pairs, they are aggressive. On the other hand, fish that are solitary in nature often can be very difficult to keep in the aquarium. That group of highly solitary fish can be the most difficult to keep together, and can even show aggression towards unrelated species.


Proper research will allow you to determine the water parameters needed for a given species. But try to also find out what the natural habitat of the fish looks like. How different is it from the aquarium landscape you are creating at home? Are the fish from a small stream full of plants, a shallow puddle, the rocky shores of a large lake, or perhaps a fast moving river? Knowing these factors in advance can be vital in avoiding problems later.


The notion that you should purchase an aquarium as large as you can afford is not a generated myth to push sales. Larger aquariums really are more stable. Water will not amass organic waste as quickly in a larger tank, and greater amounts of water are usually more forgiving in cases when the keeper makes a mistake. Keeping the stress level of your fish down decreases the chances for disease and aggression.

It’s important to remember that fish, especially in nature, do not swim up and down, but are instead tied to a stratum (and usually stick with it). In the aquarium, this translates into fish that use the bottom, middle or top third, or top and bottom half of the water column. This means that aquarium size matters much more in area (length x width) than in volume (length x width x height). In other words, an aquarium that is 6” higher or lower is less significant versus an aquarium that is 6” wider. With this in mind, I often recommend a 246 L (65 US Gal) aquarium (48 x 18 x 18) as the best bang for your buck. It’s also a size that will house most species sold in stores today within reasonable limits.

While such a large aquarium is not needed for most common community fish (i.e. tetras, barbs, dwarf cichlids or angelfish), a community tank of this size is far more interesting and will house more fish in the long-term, allowing the smaller species to grow to full size. In smaller aquariums, hormone levels and water conditions can actually delay the growth of certain fish.


Even neons and guppies create territories. A territory is usually marked by visible barriers in the aquarium (i.e. some male fish actually defend a female as their territory, without a fixed location). Barriers can include plants, rocks, wood, decorative ornaments, caves, filter tubes, heaters or even the corner of the aquarium glass. When designing an aquarium, always consider how many fish will be kept, what kind of territory they will cover (top, middle, bottom) and which fish will benefit from a cave? Ensure there is at least one cave for each fish that requires one, and that the aquarium includes divisions that can be used as territorial boundaries for each species. For the top third, this can include floating plants, decorations that reach all the way to the surface, or a cave hung from the surface on a small block of Styrofoam (great for very aggressive fish that will not allow a subdominant animal near the bottom).

Barriers should be designed in a such a way that fish occupying the middle of a territory cannot see their neighbour. This greatly cuts down on aggression and ultimately reduces stress on the fish. A rock in the corner, with space for a fish to hide behind it, can have great benefits for the aquarium. The general idea that you will see your fish more if you provide them with more places to hide is absolutely true.

There are also invisible barriers for fish. The first is the distribution of light in the aquarium. Surface plants and placement of light can create light beams and shadows, which in turn, can be interpreted by fish as a location to be defended. The second invisible barrier is water current. In nature, dominant animals often occupy the best spots in the habitat. When feeding, the best place can be the center of the current (where food is flushed down the river). For breeding and hiding from predators, the areas outside the main current are preferable. Creating a directional current in the aquarium can have the same effect. A powerhead can provide current that can be aimed at a channel between rocks, increasing the water flow in that area. The two slower moving areas to the front and back of it are then divided. For fish to cross the current would require the same effort as swimming around a rock or plant.


Feeding your fish can be entertaining, but this can also trigger aggression in their race to get at the food first. Feed your fish in different places of the tank, or feed a variety of foods in small amounts at the same time. For example, a cichlid picking at sinking pellets from the gravel will be little concerned with tetras eating flakes from the surface. Also, feed in near darkness. Different species have varied vision with reduced light. Shy fish that are dominated by greedy barbs and tetras have an opportunity to feed with the lights off just after sunset, or with only a TV lighting the room.


When fish are breeding in your aquarium, all bets are off. Species that show limited aggression will now take on very large territories, especially if they’re protecting eggs and/or fry. Be prepared to remove babies and/or parents from the aquarium in order to protect the other fish. A quarantine tank is always recommended not only to separate aggressive fish or babies, but also to act as a hospital should your less dominant species suffer any damage from aggression. For more information on the subject, read our article on the importance of quarantining new fish.

I hope you’ve found these tips helpful. I wish you great success in controlling aggression in your own aquariums. Always remember, the more informed you are before starting a new tank, the less surprises you’ll encounter down the road.


Based on your browser settings, we've redirected you to the United States English Fluval website.

Update Your Site & Language