Schooling Fish And Their Behavior

Now You See It, Now You Don't

by Robert Ricketts

In the previous article "Aggression In and Out of School" the topic of schooling centered primarily on Tiger Barbs, and close relatives of about the same size if slightly less innate aggression. There are many other families of fish that show schooling. For many Tetras, Rasboras, Rainbowfish, Danios, and even some catfish, it is equally or more important in the wild that the fish swim together in a coherent group for protection from predation. As was the case in the rather feisty Barbs, we think of these fish and picture them in our minds not as an individual fish, but as a group, a school. For their own security and health, we should keep these fish in schools also. In nature, an individual fish or a pair of fish has little chance of escaping predation, essentially none of thriving and successfully reproducing.

We want to keep these fish in our tanks. Many are small and very colorful. Most are harmless to other fish. So our first community tank is set as miniature Noah's Ark, with small Tetras and such put in two by two. The folk with larger tanks may be bold and put them in by threes. Now I will not say they that they cannot survive like this. They can. They do. I will say that they will do better and live longer and hopefully healthier if they are in larger groups. How many of us are aware that even the small Tetras such as Neons and Cardinals, who are all but annual fish in the wild, in tanks should live at least four to six years? Not have an odd survivor that makes it to four, but the average for the whole group should be that? Freedom from predators and from disease, high water quality and regular (but not excessive) nutritious food allows them to exceed what their average lifespan would be under natural conditions. One of the great revelations of fishkeeping that was pointed out to me a number of years ago was that good aquarists are easy to spot- the inhabitants of their tanks do not change. They tend to have the same fish every time you visit. They have found what the fish they keep need to thrive, and they give them that. It is not a very subtle concept, but I was floored. We can do a lot better than Mother Nature, because we do not have to provide the whole food chain or a balanced ecology. We want one frame out of the whole movie. You can say I want to keep this Tetra. You can research the fish and see that they live in water with a certain chemistry, the habitat looks like this or that, and they eat these foods. A habitat tank may not be what we want, but we have information that we use to set a tank in which they can thrive, not just survive.


When the fish we want are by nature schooling, some minimum number is needed to make the individual "feel" safe. When their conspecifics are always in sight, the world is likely to be safe. In the wild, small schools are the exception. Numbers in the hundreds are expected. Not being Dr. Doolittle, we cannot ask them how many are enough. Two is unlikely, three may be 50% better than two, but still too few. I would pick six as the minimum number, twice that if possible. This is one of the rare cases where more IS better. Given the average size of tanks, the higher numbers are not likely to be accepted by many fishkeepers, but will you at least consider six? This is enough for most small fish in small to moderate tanks to keep each other in sight, and so be under lower stress.


Now for the discouraging part: You have accepted the advice given above. You set your 20-high with six Corydorus catfish of the same variety, eight Cardinal Tetras, and six Lemon Tetras. You are going to get to watch three distinct schools moving around the tank, right? Probably not. There are two somewhat independent reasons for this.


The first reason is relative swimming room. This is the swimming space available in relation to the length of an individual fish. If the open swimming space is not a large multiple of the fish length, there simply is not sufficient space for the individuals to coordinate their movement. By the time the majority (more than half) of the fish have fallen into pattern, the leaders will have been forced to turn and start back. The result is chaotic movement within the group, as individuals within the advancing pattern sense the return of the leaders and themselves reverse direction, or perceive the abrupt reversal as indicating threat ahead and break for cover in another direction. A secondary influence on this is the presence of other fish scattered in the path of the school, forcing detours and thus less than the coordinated movement that we would see as schooling.


The second reason is that schooling is a response to perceived threat. It the wild, predators are to be assumed, a given, and the individuals in the school stay on alert. In our tanks it requires no more than a few days for the fish to "learn" that the threat level is very low. Tight cohesive formation is unnecessary. So they scatter here and there, with conspecifics in sight, but no more than that. So we delight in seeing the school act like our idea of a school the first few days, and after that, rarely. Corydorus cats are especially good at that. In fish stores we see them moving back and forth in the tank as a group, a great sight. We do not realize that this is prompted by the fact that they have only been in the tank a day or so, and there are giants, potential predators (us) peering at them from only a foot or so away. Come back a week later. The same group of cats is sitting around the bottom, largely ignoring the giants going by. They have learned we are not predators after all and that there is no way to swim away. Open the tank top, and they start swimming again. The only predator they have seen lately, the net, is on the way. An immediate threat activates flight behavior. The influence of learning on instinctive behavior is obvious.


So what is the solution? There are several, depending on what you are willing to do to keep and observe schooling. The best for me is to increase the tank length, and with it, the open swimming space. Has everyone seen the photographs of Takashi Amano's "plains" tanks? These are large tanks having the substrate covered with low-growing plants and only a few outcrops of rock or a clump of foliage-covered wood interrupting the rolling plain. There is relatively little cover. The small Tetras or other fish would feel insecure in such open space, but they have safety in numbers, and so they school. Gloriously. These tanks are in no way Noah's Arks. They are species tanks. One species of fish predominates, and are present in sufficient numbers to offer security in a real, almost natural, school. Other creatures may be present- commonly in these tanks we see Otocinclus cats (who also school) and shrimp, but none that will disrupt the primary school consistently.



We cannot all manage (or afford) an Amano-style tank, but we can apply some of the principles. Select a relatively long tank rather than a tall one, say a 20-gallon long rather than high. Keep the tank open, using plants or wood or rock only at the back corners, and control the size of the school relative to the size of the tank. Add a dozen Neons or Cardinals rather than twenty. This will lower the interference of sheer numbers in a small space. Add no other fish that is usually free-swimming. You will see schooling. Do not try this with the larger Tetras, Barbs, Rasboras, Rainbowfish, etc.-- there is not enough relative swimming space. For them you must increase the tank size. You could try a 30-gallon long (36") for the medium fish from these groups, or move to a 40-gallon tank (48"), or a 55-gallon for the larger active midwater swimmers.


In the larger tanks using small fish, say Harlequin Rasboras in a standard 55 gallon tank, or the comparably-sized bow-front, you can boost the perceived threat by adding a small number (perhaps a male and two females?) of fish large enough to appear a threat without being one. Perhaps some Pearl Gouramis, or Moonlight Gouramis would serve? Plant this taller tank heavily with low-growing Cryptocorynes, and a few clumps of the taller members of the same genus at the back or corners, but not lined up like a hedge. Now there is a tank to lower your blood pressure- and to impress your spouse and friends. There is great beauty here in grace, style, and color. There is peace in smooth flowing movement, and for me, immense satisfaction in the jewel box that has been created.