Related FAQs on: Freshwater Behavior Related Articles: Chemical Communication in Fishes, Aquaculture,  The School of Hard Knocks. Adapting Their Behavior To Life in the Aquarium Isn't Easy, But Many Fish Manage To Do So Remarkably Well
By Neale Monks

It isn't uncommon to read articles in aquarium magazines about fish behavior. A lot of aquarists will be familiar with the sophisticated broodcare extended by cichlids to their offspring. Others will be familiar with the territorial behavior of Bettas and the way male mollies spar with one another for access to females. Some lucky aquarists will have seen archerfish spit for food and mudskippers walk on land. But what I'm more interested in here is how fish behavior changes in the aquarium. Can different species of fish communicate with one another? Do fish really become tame? Does their behavior really change, or are they doing in an aquarium pretty much what they'd be doing in the wild?

 

What's normal behavior?

To see the changes, we need to establish the baseline. There are many behaviors that are fundamental to what fish do and don't normally change very much at all. Many fish like to live in groups, both in the wild and in captivity. Corydoras catfish, for example, live in schools comprising hundreds of individuals, and in captivity they are certainly a lot more outgoing and settled when kept in groups than singly. Sociable fish often have hierarchies that define the status of each member within the group. Extreme examples of this can be found among the marine Clownfishes, where the biggest fish in the group is the female and all the smaller individuals males. This isn't because the male and female fish come together to form such a group, but because all baby Clownfishes are males, and only if one of them becomes the biggest and most aggressive fish in the group does it change into a female.

Even where fishes don't change sex, there will often be a definite pecking order in the group. From our perspective, it isn't always which fish is in charge, but watching the fish sparring with each other as they establish and maintain that hierarchy is easy enough. Tiger barbs chase one another, tetras flick their fins, Firemouth cichlids flare their gill covers, and halfbeaks wrestle. But as well as having ways to pick fights, schooling fish need to be able to stop fights too, not least of all because the survival of the group depends on all its members being equally alert and agile. Often, the weaker fish displays a signal that makes it clear that it accepts the higher status of the bigger fish within the group. Many cichlids use color for this, such as dark vertical bands on the flanks that advertise to aggressive males quite clearly that the submissive individual is not a threat. Many of the schooling, open water cichlids of Lake Malawi show this very well, as do discus and angelfish.

Of course, not all fishes live in groups, and many are intensely territorial, driving away other members of their own species from their home range. A lot of damselfish, Bettas, cichlids, and Loricariid (Suckermouth) catfish are animals of this type. Territoriality serves a variety of functions. In the case of damselfish, the territory-holding fish try to monopolize Gardens of algae that they cultivate as a dependable source of food. Male Bettas and cichlids, on the other hand, are guarding nesting sites that they will use to attract females and raise their young.

Crossed wires

Schooling behavior especially seems to go wrong in aquaria under certain circumstances. Where fish get along peacefully in groups of six or twelve when kept in twos or threes they can often be very aggressive to one another. Monos, archerfish, angelfish, and piranhas are among fish species that can be very problematical when kept in too small a group. It seems to be the hierarchy that fails by not being spread out evenly over a large enough number of fish, with the result that the dominant fish ends up bullying the one or two other fish in the group, often to the point of death by preventing them from feeding properly. With other species, this aggression spills over into the rest of the tank, and the fish become nippy and waspish towards their tankmates. Tiger barbs are the classic examples of this, being fin-nippers when kept in small groups but usually much more peaceful when kept in a group of a dozen or more.

Something that fishes in aquaria have to deal with that they don't in the wild are the full range of interactions with other species of fish. Wild fish will have to compete with rival species for food or nesting sites, and small species will need to avoid being eaten by larger ones, but within the aquarium, these relationships are taken to a whole new level because of the lack of space and high stocking density. Where dwarf cichlids like kribs may have several square feet of territory to work with in the wild, in an aquarium the same area may be home to several pairs of fish, not to mention assorted catfish, loaches, and other bottom-dwelling species.

Corydoras catfish in particular seem to have a very bad time with dwarf cichlids. In the wild, Corydoras live in creeks and streams, typically moving across the riverbed as a large group, replying on their schooling behaviour and stout body armor to keep them safe from predators. Dwarf cichlids, and indeed most other small, territory-holding fish, tend to live in sluggish waters, particularly pools and riverbanks where the caves and burrows are more abundant. The poor Corydoras catfishes just doesn't seem to be able to get their heads around the concepts of territories and boundaries, and blithely swim into patches held by dwarf cichlids and end up getting nipped and chased as a result. In a small aquarium, mixing Corydoras even with cichlids as otherwise benign as Apistogramma and Pelvicachromis can be very unwise. Loaches and Loricariids on the other hand not only understand territories, but hold territories of their own as well. As a result, both tend to work rather well with cichlids of comparable size, quickly establishing an armed truce where each fish respects the boundaries set by the other.

Midwater fish will also have to find ways of getting along with species they would never encounter in the wild. In my own community tank I have some glassfish and silver hatchetfish, the former from Asia and the latter from South America. Strangely enough, they can and do interact. Perhaps their shape and coloration is similar enough that they treat each other as equals? Certainly their mode of establishing a hierarchy Ã' best described as dive-bombing one another Ã' appears to translate pretty well, and the two species will alternate between short chases and schooling freely apparently smoothly and without harm to either party.

Sometimes though, there are gaps in communication that make it difficult or impossible for the two species to communicate with one another. An example of this occurred in a slightly brackish aquarium I kept many years ago, within which were some sailfin mollies and a group of large Melanotaenia rainbowfish. Rainbows are from Australia and mollies from the Americas, so again, this is a combination that wouldn't occur naturally. The problem was the high, arched back of the rainbowfish was too similar to a male sailfin molly with its dorsal fin erect. The male mollies would swim around the rainbows, raising their fins, and generally being aggressive and annoying. Male mollies can of course lower their dorsal fins when they're done fighting, but the poor rainbowfish couldn't do anything to diffuse the situation but swim away, and this only worked until a rainbow and a male molly bumped into each other again.

On the other hand, I've seen combinations where inter-species communication doesn't just fail it; makes things worse. Colombian shark catfish, Hexanematichthys seemanni, thrive in marine tanks, and on once occasion I put a trio of these catfish into a large marine aquarium alongside a variety of species including a small blue triggerfish, Odonus niger. Blue triggers are amicable enough (for triggerfish, anyway) and Colombian sharks are entirely peaceful, so on paper at least this looked good. In reality, it was a disaster. The problem was communication: both species produce drumming and clicking sounds, but they do so for entirely different reasons. Shark catfish produce sounds to help the school stick together in murky water and also to warn potential predators that they are venomous and best left alone (like the rattlesnakes rattle). Triggers use sound as a threat. As soon as the catfish started making noise, the triggerfish immediately went on the offensive, and started badgering the catfish. The more threatened the catfish were, the more noisy they became, and so the triggerfish became even more aggressive. Needless to say, the two species ended up being separated.

 

Think different

Assuming we put together a group of fish that don't eat, attack, or otherwise molest one another, the fish may come to learn to take advantage of one another. Some schooling fish will Make up their numbers by schooling with other species. Different species of Corydoras are famous for this, but many other fish will do so as well. Some of the smaller tetras will form mixed groups, including Neons, cardinals, glowlights, and black Neons. At the other end of the size range, scats and Monos will also form mixed groups.

Fishes will also learn to exploit the activities of other species, sometimes displaying a remarkable degree of opportunism. A typical example is the way small tetras and barbs will follow large but messy fish such as substrate-sifting cichlids, snatching up bits of food that the larger fish throw into the water. Rather more surprising was a scat that I kept that learned to take advantage of the hunting behavior of an archerfish in the same aquarium. The archerfish was fed by being offered small bits of prawn stuck to the glass, which it would spit into the water. After a while, the scat learned that when the archerfish began spitting, a free meal wasn't far away, and over time it got increasingly good at racing to the downed prawn before the archerfish had a chance to get to it.

Some fish will learn to do things that in the wild they would never do because it would either be too dangerous or simply impossible. Gobies normally swim close to the substrate and rarely far from their burrows, but in aquaria, gobies often become much more adventurous. The popular and colorful candy-stripe goby, Awaous flavus, will happily perch on floating plants looking out for food, and my specimens would swim onto my hands if some tempting bloodworms were put on offer. An even more remarkable example of this type of behavioral flexibility can be seen when fishes that normally stay in open water decide to explore caves and burrows. In the wild, this would be very hazardous, since the survival of these fish depends on them being able to see predators and swim away quickly at the first sign of trouble, something they cannot do among rocks and bogwood caves. But once settled in, these fishes seem to learn that there aren't any predators in the tank, and that they are free to explore the whole tank in safety.

Sometimes, their body shape makes this sort of behavior rather comical. I have one particularly inquisitive Celebes halfbeak, Nomorhamphus liemi, which has learned to stick her head into caves and cracks to winkle out overlooked bloodworms. Of course, her beak gets in the way, as evolution equipped her for taking food from above, not below, so she needs to tip over to one side to snatch up the food. Her arrangement of fins doesn't help either, and as good as she may be at straight-line sprinting, she's absolutely hopeless at shunting in and out of crevices.

Our world through their eyes

What is even more fascinating is how fish learn to take advantage of us. For supposedly stupid animals, fish are often very quick to learn and many species can become completely tame. While this is fairly well known to be the case with big fish like Oscars, even small fish can be tamed. I have some wrestling halfbeaks (Dermogenys pusilla) that come to the front of the tank as soon as I open the hood. If I hold bloodworms with some forceps, they will snatch them up one at a time, and I can even gently stroke their backs with a wet fingertip! Predatory catfish, pufferfish, spiny eels, and goldfish are among the freshwater fish that become tame quite easily, and in the marine tank puffers, triggers, groupers, and damselfish are equally adaptable.

As with training any animal, repetition and reward are the keys. Animals don't like surprises, which they treat as a threat or possible hazard. But once they get used to you doing something at the same time, day in, day out, they become much more trusting. I have an eleven-year old Panaque nigrolineatus in the tank next to my dining room table. At lunchtime I have my sandwich there and watch the fish. As I sit there eating, the catfish becomes agitated, leaves her cave, and swims to the front of the tank. Now understand that Panaque nigrolineatus is ordinarily a shy, nocturnal species, but over the years this old girl at least has learned that this is a good time to get food. Of course she hasn't learned that I'm the salad-fairy, but she has learned that when she sees me at a certain time of day, then there's a good chance that some food will be forthcoming. Who says you can't teach an old catfish new tricks?

Many fish, perhaps all of them, have a basic drive to explore their aquarium, and by adding decorations and the right tankmates, the aquarists can create an environment where each fish is stimulated and entertained. When animals like dogs and cats get bored, they become destructive or aggressive, so why shouldn't the same rules hold for aquarium fish as well? Pufferfish spend their day searching for p crabs and clams that are normally very well hidden. Before they find a single meal, they'll have to inspect dozens if not hundreds of square feet of rocky reef. So simply giving them a defrosted prawn isn't going to engage them. Some people using 'feeding stones' to keep them occupied; lumps of lava rock into which food is smeared and the pufferfish has to carefully extract small pieces one at a time from the tiny crevices in the rock. Of course, the pufferfish don't usually get all the food out, so after an hour or so, you'll want to pull the rock out and clean it off. Fish are really a good deal smarter than we give them credit for, and understanding this goes a long way towards making their life in an aquarium more interesting and our hobby more rewarding.