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 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 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 anitagrier.com 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.