Fish & Their Environment

Fish have been on Earth for about 500 million years and counting. It isn’t surprising as they quickly adapt and thrive in numerous environments, from oceans and open seas to lakes, rivers, and streams.

What do fish need to survive in their environment? Like all living organisms, fish must meet basic survival, growth, and reproduction needs. These basic needs are water, food, and shelter, and these are must-haves no matter where they live.

Below is a list of the things required by fish to thrive and survive in their environment. Along these requirements are further explanations on why they need them.


Fish live in water, where they find mates, give birth to their young, and find food, among others. They also require water for their oxygen, the same way humans need the atmosphere for oxygen.

But fish have a different way of breathing in oxygen than humans. While we breathe through our noses, fish breathe by taking water into their mouths.

They then force the water through their gills, which have a network of tiny blood vessels spread over its surface.

Fish have a unique respiratory system that allows them to spend their lives in water. Most fish will suffocate from lack of oxygen when they are on land, even for a short period, and then die.

But there are exceptions to the rule since fish can live on land, too, sometimes for short periods, sometimes for extended periods.

Known as amphibious fish, they have developed adaptations for on-land living, such as tripod-like walking, swim bladders, and eyes suitable for seeing clearly in the air. These include eels, snakehead fish, and rock skippers.

Most fish also live in one of two habitats based on salinity, namely, freshwater and saltwater. For example, goldfish can only live in freshwater, while tuna can only survive in saltwater.

But there are fish species that can live in both environments! Euryhaline fish can withstand significantly different salt levels and even migrate between freshwater and saltwater.

They are further categorized into:

  • Anadromous Fish – These are born in freshwater but live most of their lives in saltwater. They return to freshwater when spawning, as is the case for salmon, shad, and smelt.

  • Catadromous Fish – These mostly live in freshwater but go into seawater for the spawning season. Examples include the European eels and North American eels.

Water is, of course, also crucial for the movement of fish. Most fish move faster and easier in water because of their fins and tail but can’t move on land because of the legs’ absence.


Fish should have sufficient food to eat at their various life stages. Their diet is influenced by their life stage, environment, and food availability, among other factors.

They can also be classified according to the main types of food in their diet, such as herbivorous, omnivorous, and carnivorous.

The diversity of marine life also means that fish eat a wide variety of food, from phytoplankton to fish.

Phytoplankton is placed at the bottom of the ocean and seas’ food web! Think about it: Phytoplankton is eaten by krill, then eaten by nearly all fish and mammals in the sea.

Other food sources for fish include:

  1. Herbivorous fish prefer seagrass and algae.
  2. Detritus are produced when fish release solid waste into the water, combining with other organic matter and coral slime. These accumulate on the seafloor and become food for some fish species.
  3. Sponges are a favorite food among angelfish, omnivorous fish.
  4. Crustaceans, such as crabs and shrimp, are also a popular fish food.
  5. Invertebrates like snails, worms, sea urchins, and starfish, as well as corals, polyps, and clams, are also favorite food sources.
  6. Parasites that live on certain fish species serve as food for other fish, too. The janitor fish comes to mind.

Of course, fish also eat fish! As can be expected in the wild, large fish eat small fish, a fact of life that maintains the balance in the seas.

In aquariums, the diet of pet fish is quite different from their counterparts in the wild. Their food usually consists of pellets, granules, and flakes, as well as peas, zucchini, and raw potatoes, to name a few.

The specific type of food will vary depending on the species, such as angelfish feeding on brine shrimp and betta fish on bloodworms.

Tip: When deciding the types of fish that can be placed in a single aquarium, be sure to consider their preferred food and their behavior toward other fishes.

For example, angelfish will likely eat smaller fish like tetras, while betta fish can live with other fish if they are more peaceful.

Before setting up the Marina LED Aquarium Kit, it would help if you also considered reading informational books, such as The New Aquarium Handbook.

You don’t want the fish-eating each other lest you end up with just a single predatory fish in the tank – unless that’s your goal, in the first place.


Fish also need shelter but not a roof over their heads, as we know it. Instead, it’s the kind of shelter that will provide a hiding place from predators, as well as provide a place for reproduction and rest.

Fish in the lower rungs of the food web will need shelter more often than the apex predators.

There are many places under the water that fish will consider as shelter. These include aquatic plants, such as kelp, shoreline vegetation, soft sediments, rocks, and hollow logs.

Even the detritus of human society, such as plastic trash, Christmas trees, and sticks (usually placed by anglers to attract fish), can serve as shelter.


To conclude, fish are amazing creatures that have developed adaptation characteristics to thrive underwater where a man cannot.

But they also need food and shelter to survive, just like us.  With millions of years in evolution, they will continue living on Earth, perhaps more than we ever will.