The fish’s basic "intelligence" and sensory skills are adapted very differently from the land-based animals. The animals’ primary concerns are, fundamentally, to eat and not be eaten. Aquatic dwellers also employ their senses mainly for self-preservation and the propagation of the species.
Fish sense hunger, pain, and threat. They become aggressive when biological triggers signal the craving for food, or the need to defend a brood or a territory. Fish also communicate with one another, and may even try to communicate with you. In order to begin to understand what your fishes are "saying," here’s an introduction on how they function and interact with the environment.
Sight
Optimized to see food, partners, and potential predators, different types of fish eyes have evolved to adapt to the environment. Nocturnal fish have large eyes; muddy river dwellers have small eyes; cave dwellers are totally blind.
The eyes of mid-water fishes are usually located on the sides of their heads to give them a 360-degree view of everything around them. The eyes of bottom dwellers, on the other hand, are usually located on the top of their heads to catch all the action above them; while those of surface feeders have evolved into split focal vision to see both above and below the waterline.
Shortsighted compared to terrestrial creatures, most fish have monocular vision. This means that each eye can focus on an object independently of the other eye, affording them an all-around panoramic field of vision. On the other hand, most predatory fishes have binocular vision with eyes close to one another and in front of the head which, like in humans, allows an accurate assessment of the exact location of their prey.
Some shallow-water fish are equipped with false eyelids that they can lower to shield their eyes from bright sunlight. The majority of fish, however, do not have the benefit of these "shades." Because of this, it normally takes them a while to adjust to sudden changes in lighting, and it is best to wait a few minutes after turning the aquarium light on before feeding them.
As for color vision, research has confirmed that fish can see color, to some extent, above and beyond some practical uses of color. For example, the fact that most strikingly colored species use their pigmentation to attract mates; or use color to hide from and ward off predators.
Hearing
Sound travels more than four times faster through water than through air. Even without a middle and outer ear like other animals, a fish can "hear" sound through the vibrations in the water, via a porous lateral line along the sides of its body. By picking up water vibrations, the fish becomes aware of feeding frenzies or fearful fleeing that happens nearby.
Aside from this sensory lateral line, most fish have an inner ear, called an otiolith, which functions very much like the human eardrum. It is usually located at the back of the skull, and like the human inner ear, it also helps the fish maintain orientation and balance through cilia that send messages to the brain.
Therefore, because fish can sense the smallest vibrations through the water, tapping the aquarium tank, the stand that holds it, or otherwise causing sound waves to travel through the water, can cause a great deal of shock, stress, and torment to the resident fish.
Touch
The sense of touch is highly developed in fish. This is not because it helps them differentiate textures or express emotions like other animals, but because it is closely associated with their sense of taste, direction, self-preservation, and territoriality.
Gouramis and Angelfish use their pelvic fins to taste and grope for food, but they also use their feelers as warning devices when coming out of a hiding place. They use their fins to feel their way around and, at times, to assert dominance during a territorial dispute. Catfish and most nocturnal species use their barbels to sense their way in the dark, to dig through the substrate, as well as to taste their food.
Taste
A fish’s taste buds are located in receptors that are spread on several areas of the body. Many fish have receptors on their heads, mouths, lips, and on special extensions, like the barbels on the catfish. Many species have taste receptors on the skin, which sends food messages to the brain, and prompts the fish to swim towards the source of the food taste.
Goldfish have taste sensors inside the mouth – when the brain interprets a particle as inedible, the object is quickly expelled.
Smell
Many varieties of fish have nostrils with which to smell odors emanating from food and mating partners. These nostrils have sensors that detect odors from the water and send signals to the brain. Some of the more sensitive fish, like catfish, and those with poor eyesight, have connecting pairs of nostrils that allow water to pass into the front nostril and flow out the rear opening.
Not only do these sensors help the fish taste and zero-in on the food, they also function as receptors for the "odors" that some fish emit into the water to denote position and rank within a community or territory. Most females also emit scents when they are ready to mate.
Be Sensitive to Fish Sense
Aside from behaving in response to the natural stimuli in their tank environment, fish have also been known to respond to their keepers. Larger species, like the Oscar, Pacu, Grouper, Arowana, and Triggerfish, are receptive to hand feeding; they express hunger by following the movements of their owners.
If you are observant, you will also notice that fish behavior can alert you to impending problems in your tank. Peaceful fish that suddenly show aggressive behavior towards tank mates and aquatic foliage may be signaling stress or insufficient feedings. Territoriality may signal that a fish has reached maturity and must, therefore, be given a new set-up. Unusual behavior, like hiding, refusing to eat, scratching, and erratic swimming are strong indications of health problems.
Your awareness and concern for your fishes instincts, senses, and means of communication will ensure that the community remains healthy and viable. Here is a summary of reminders:
1. Provide the appropriate lighting for all your fish. Avoid sudden changes of light that could shock or stress, especially the species that are nocturnal or highly sensitive to illumination.
2. Never tap the aquarium glass. Sound waves are magnified by water and the noise can be stressful to the fish.
3. Provide the appropriate substrate for fishes that use barbels or extended fins to taste or grope through the ground. Jagged gravel and sharp rocks can damage the fish’s feelers and taste receptors.
4. When transferring fish from one location to another, use a soft net to minimize harm or damage to the fish’s skin, fins, or orifices. Any damage will affect the fish’s natural capabilities to sense and respond to its environment.
5. Be attentive to "body language" messages that your fish may be communicating, and respond accordingly.
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