The Division of Tanks – What and When

Whether it’s due to aggression, territorialism or just plain old size difference it’s pretty safe to say anyone who keeps more than one non-schooling fish in a tank for a long time is going to need to separate them somewhere along the line. Many people don't have the room or the funds for a specific tank for this purpose and that's when alternative techniques come into play.

When dividing tanks one of two approaches can be taken; 'physical' OR 'behavioural'. The idea of this article is to familiarise you with techniques form both of these approaches and the situations in which they should be used.

Reasons to Divide a Tank

First off let’s discuss the reasons for dividing fish as this will help decide which approach to take.

  • Aggression; Aggression in aquariums happens for many reasons the main being that the fish just don't get along either because their species are not compatible or just as common their personalities clash. Two scenarios in which this aggression could occur are; fish such as gouramis and betas (who exhibit aggression towards each other consistently) or a convict who just doesn't like other fish because that's just the way he is.
  • Territorialism; Territorialism or; a fishes need to have its own or their own (in the case of a breeding pair) space, in which no other fish are allowed, is also one of the major reasons for separating fish. This is often seen in the case of breeding pairs who in an attempt to protect their young protect their breeding site.
  • Size difference; bigger fish will if they are inclined, eat smaller fish so if you want the smaller fish to keep on swimming you might just need to separate them.

How to Divide a Tank

Now that we know WHY to separate fish, let’s talk about HOW to do it. As stated earlier, their are basically two techniques. These are;

  • 'Physical' or the inserting an actual barrier to separate the tank into two separate areas. Physical separation can be used in all of the above situations
  • 'behavioural' or the act of using the fishes natural behaviour to effectively divide the tank by 'separating' the tank using decor and substrate to define ‘boarders’ allowing fish to establish different territories. Which is used in situations such as a breeding couple, territorial fish (remembering every tank is different), but is usually unsuitable for unbridled aggression and size difference.


We will start by addressing physical separation techniques, physical techniques are arguably the most useful as they can be applied to any scenario. Physical dividers consist of a dividing surface and a method of attachment. To familiarise you with the techniques used in physical tank division I’ll run through all of the popular dividing surfaces and the popular attachment techniques.

Dividing surfaces

The biggest and most important part of a physical divider is the dividing surface. There are a few choices when it comes to dividing surfaces. There are also many things to consider when deciding on which to use keep in mind the; size of the fish, the size of the tank, the ability for water to flow and be filtered and the transition of heat throughout the tank. There are tried and proven dividing surfaces, the main three being egg crate, plastic canvas and plexiglass each of which has its advantages and disadvantages and suitable uses.

Egg crate

Courtesy of wickedLESTOR

Egg crate is cheap and easy to work with, it is also very effective. Egg crate is a ‘large mesh’ made of plastic, and is available at all INSERT PLACE. Simply cut to shape and attach to the inside of the tank to effectively divide it (attaching dividing surfaces will be discussed later). It is easy due to its structure to link multiple sheets of egg crate together or attack things to it.

Plastic Canvas

Courtesy of Ravyn_Smoke

Plastic Canvas is the little brother of egg crate, it is a finer mesh available at art and craft stores.


Plexiglass is exactly that a pane plexiglass cut to size and holes drilled into it. It has a few advantages over egg crate; it’s sturdier, harder to see and the sizes of its holes are customisable, this is where the advantages end. Plexiglass is harder to work with, requiring power tools and some level of technical knowhow; it also has very limited options in attachment techniques. Due to this it is rarely used as a divider, only in cases when a stronger divider is needed.

Attachment Techniques

Attachment techniques are the way in which you connect the dividing surface to the tank itself. Much like the dividing surface many variables must be taken into account when deciding on which attachment technique is to be used in a specific tank, the main two being wether the barrier is to be permanent or not and the strength of the fish involved, after all you wouldn’t hold a bull pen together with twine.


The first, and simplest, attachment technique is ‘propping’. This is when you simply cut the dividing surface to size and use the substrate/decorations and the sides of the tank to prop up the dividing surface. ‘Propping’ obviously isn’t permanent. It is also unsuitable for use with plastic canvas (as it bends). Propping should only be used in tanks containing fish that do not posses the strength to disturb the dividing surface. It is the most unsafe attachment technique.

Rubber tubing

The use of rubber tubing is in-fact an improvement on the ‘propping’ technique. It’s a simple concept the dividing surface is cut to size and two pieces of rubber tubing are slit down the side and the edges of the divider placed inside them. The divider is then inserted into the tank (the rubber sides holding it up and preventing the dividing surface from scratching the tank’s glass. This technique is suitable for use with all of the dividing surfaces, and all but the strongest fish.


Framing is yet another improvement on the original ‘propping’ concept. The general idea is a frame (consisting of a square frame around the dividing surface and a structure that allows it to be free standing). For the dividing surfaces that are net like in structure (egg crate and plastic canvas) this can be connected with cable ties (or zip ties), for plexiglass (if truck ties cant be fitted to the drilled holes) you have to be a little more creative. Plexiglass may be connected to a frame but cutting slits into the PVC and fitting the plexiglass within. When uncovered PVC frames can be unsightly but using substrate and careful décor placement, PVC frames can be almost completely hidden. Depending on how wide the stand part of the frame is it may be impossible for a fish to move it, making the framing technique the strongest of techniques (along side silicon) and subsequently not only suitable for all dividing surfaces but also for all fish, commonly kept in our aquariums.

Suction cups

They hold your heater in place, they hold your thermometer in place, why shouldn’t they hold your divider in place? Suction cups are a good attachment technique, they can be attached to dividing surfaces with truck ties or wire and allow them to be stuck to any glass surface. The power of multiple suction cups make them suitable for most tanks (excluding ridiculously strong fish e.g. pacu).


Silicon is the most permanent attachment technique and along with framing one of the best in terms of strength and stability. When a tank is destined to be forever divided a permanent solution, that is to say silicon is the best route to take. Not only is the use of silicon one of the most reliable techniques it is the neatest and most ‘professional’ looking. The colour of silicon can be matched to that of the tank joins and with good workmanship it may be almost unrecognisable as an ‘aftermarket’ edition.

Once the aspects of the specific tank are taken into consideration and the dividing surface and attachment technique selected it’s a simple matter of assembling the divider and inserting it into the tank.


Now onto behavioural division, it is not as cut and dry as physical separation; it often takes more than one try for the separation to work, this makes using behavioural division less convenient when compared to physical division. Behavioural separation relies on a fish’s basic instinct to create a divide within the tank, by allowing each fish to claim its own clear territory, but as we have all seen a fish’s behaviour is often unpredictable and this adds to the problems associated with behavioural division. There are two different techniques when dividing a tank using a fish’s behaviour and depending on which species of fish you are stocking the way you go about it differs, there are two types of behavioural division one which uses structure to create the boundaries and one which uses space to create the boundaries.

Courtesy of SandTiger

Using Structure

Using structure to create boundaries is like using walls to mark your property the fish’s territories consist of open spaces with ‘walls’ separating these spaces. ‘Walls’ can be made from anything from rocks to plants. This technique is most effective for non-cichlids.

Courtesy of SandTiger

Using space

Using space to create boundaries is like the ‘no-mans land’ that marks the boundry between two trenches. The fish’s territories consist of décor and substrate formations between which there are areas of open space. This technique is the most commonly used on Oscarfish as it is the most effective technique for use with cichlid species.

Using cover (caves and plants)

There is a third behavioural ‘separation’ technique, that is, the use of cover. This is the only behavioural ‘separation’ technique suitable for use in SOME cases of unbridled aggression and size difference (always remember EVERY TANK IS DIFFERENT and while it is suitable in some cases it is still much safer to use a physical separation technique). The general idea is to provide an area for the less aggressive/dangerous fish to retreat to when being attacked/intimidated, of course you must take the size, strength, species, and speed of both fish involved into consideration. Giving a betta some plants in a jaguar tank isn't going to keep it alive, but in the case of an Oscar and some fast dithers you would have much better results.


Medical assistance (hospital tanks and their benefits)

Hospital tanks aren’t really a tank division but they are a handy tool in fish keeping, and so I’ve decided to give them a brief mention. The basic idea is a smaller tank in which a fish can be placed while they heal. This protects them from dangers such as aggression or infection by other fish, and allows treatment to be performed easier and for less money (as there is a smaller quantity of water there is less medication used). Important points to remember when setting up a hospital tank; Filtration must be considered (full mechanical and biological filtration must occur to prevent dangerous chemicals from occurring in the water column), many keep cycled media to use in such a situation. Remember when using hospital tanks water changes, water changes, water changes! As a hospital tank holds less water than the original tank, nitrates build up more quickly. Finally there is no need to decorate a hospital tank, your fish definitely won’t care, and all it will achieve is wasted space which instead should be dedicated to much needed water.


Not really a method of tank division, because it involves a seperate tank. But sometimes, no matter what you do, two fish simply may not coexist. When this happens, the only option is to place the fish into different tanks. For those of us hardcore enthusiast with spare tanks laying around, this is not a big issue. For most, however, this involves returning a fish to the LFS, finding a new home, or purchasing an additional tank. For extreme cases, where one fish's life is in danger because of aggression, there are various “storage containers” (Rubbermaid makes some excellent ones) that are inexpensive and make fanatastic temporary tanks.