Fish have many well developed senses some such as sight, smell ect which are familiar to us and some like the sense which comes from their lateral line which we can only try to imagine. Fish rarely rely just one sensory system and can change their responses to events using several sensory systems at once depending on the demands and the sensory environment in which they are in.
The sight of fishes varies enormously, some species have excellent vision while other no longer use sight at all like the subterranean characins of Mexico which although born with eyes they are quickly covered by skin leaving the fish completely blind. For most fish which live in well lit waters vision is a very important sense for both locating prey and for avoiding being prey. Most fish which live at sallow depths where light easily penetrates have colour vision although some colours are seen more easily than others because even in fairly shallow water some colours penetrate better than others.
Any fish which had eyes which were adapted to see red would quickly become extinct as it wouldn't see very much in water deeper than 1 m. However fish which are coloured bright red and living in deeper water would be almost invisible. Sharks do not see in colour at all.
As mentioned vision varies with species. Species which live in more open water and have a clear view of the horizon substrate - water or air - water have eyes which are adapted to give a panoramic view of their environment without the need to move their eyes. Such adaptions make the eye less sensitive to seeing movement. Species with an interrupted view of the horizon often possess good eye mobility and often have a frontal binocular vision. Some deep sea fish which live at depths where almost no light penetrates have very large eyes which can be 50% the head length these large eyes have a very large lens which is focused on a very small retina and this adaption greatly increases the amount of light reaching the eye.
Aulonocara 'Cobue' like all other Aulonocara use sensory pores around their head to listen for prey buried in the substrate where they prey would ordinarily be hidden
Although fish don't possess external ears most species have some degree of hearing. Fish of the super order Ostariophysi which contains about 8,000 species which is 28% of all known fishes including some families which will be very familiar to fish keepers Cyprinidae, Characidae, Loricariidae, and Balitoridae have something called a Weberian apparatus.
The apparatus works by sending sound signals straight from the swim bladder, through the Weberian ossicles and then straight into the labyrinth structures of the inner ear. Most sound would go unnoticed but because the swim bladder acts as a resonating chamber the sound waves are greatly amplified. So sensitive is the hearing in some fish that they are able to detect the approximate distance and direction of the source of the sound waves. Most fish have a hearing range between 20 Hz to 1000 Hz compared to humans 12Hz to 20,000 Hz fish have restricted hearing.
The lateral line runs along the length of a fish's body and can easily be seen in most cases. The line is made up of a series of pores each containing a hair like structure (stereovilli) attached to a nerve ending. The lateral line is able to sense minute changes in pressure. This effectively allows the fish to "feel" at a distance. By feeling the difference in each pore the fish is able to tell the direction and distance from the source of the disturbance and this will warn them of any predator trying to sneak up on them even if they don't see it.
Blind cave fish rely extensively on their lateral line
Surface feeding fish are able to locate prey such as flies which have become trapped on the water surface using their lateral line while blind cave fish are able to successfully compete for food in a busy community tank using just their lateral line.
The sense of smell is very well developed in some fish and is better not only than ours but it is better than in most mammals. Other species of fish like the family Tetraodontidae have completely abandoned the sense of smell altogether.
Tetraodontiformes like this Pufferfish no longer have any sense of smell at all.
The olfactory organs are located in two pockets on either side of the fish's snout. The pockets are lined with nerves which are highly sensitive to dissolved substances in the water. Fish have two nares on each side of the snout, water enters the first and passes over the sensory nerves and then leaves via the back nare so that the fish is able to pick up any scent dissolved in the water as it swims. The nares are not used for breathing like in mammals.
Like some other gouramis the pearl gourami has some taste buds on their long pelvic fins and they can often be observed touching various items in the tank using these fins.
Most fish have taste buds located in their mouth but these taste buds are not exclusively found in the mouth. Taste buds are found on the gill arches, gill rakers, barbels and even some fins but they are absent on the fish's tongue. In cyprinids and siluriformes taste buds cover the entire body of the fish with up to 300 taste buds per mm².
Like all sharks the black tipped reef shark are very sensitive to minute changes of voltage. They are even able to detect the electricity produced by a heartbeat of their hidden prey.
The Ampullae of Lorenzini is a series of pores found mostly on the head of sharks. Each pore is filled with a special jelly like substance this Jelly like substance is a glycoprotein based substance with the same resistivity as seawater, and it has electrical properties similar to a semiconductor. There are special receptors in each pore which are called afferent fibers which transmit information to the nervous system.
This is a sense which we can only imagine. The Ampullae of Lorenzini can pick up minute changes in voltage. So sensitive is it that a difference in voltage of just 0.000,000,005 of a volt can be sensed. Sharks and rays use this sense to find prey which has buried itself in to the substrate in an attempt to hide. All living things use electricity to power muscles and it is this slight change in voltage around all living things which sharks and rays are able to sense.
Some other fish have this sense but it is less well developed than in sharks. The other fish include Reedfish, Sturgeons and lungfish.
Subterranean: below ground, under the earth,
Binocular vision: is vision in which both eyes are used together.
Weberian apparatus: an anatomical structure that connects the swim bladder to the auditory system in fishes
Ossicles: simply means 'small bone'
Afferent fibres: Any of the nerve fibres that convey impulses to a ganglion or to a nerve centre in the brain
Ampullae of Lorenzini: are special sensing organs called electroreceptor's
Nare: is the nostril of a fish.
Stereovilli: Mechanosensing organelles of hair cells which respond to fluid motion
Wikipedia - Ostariophysi
Wikipedia - Weberian apparatus
Georgia institute of technology
Earthlife.net - Fish - smell
Think Quest - Senses of fish
About-fish-online - Anatomy
Gross functional anatomy: Chemoreception. Pages 246 - 247