How fast fish grow will depend on their species. Some species grow faster than others. There are over 34,000 species of fish so you would probably have that many answers to this question. Also, there are other factors to consider such as the temperature of the water and quality of the water, their genetic factors, and more. For example, with the right conditions, a bass can grow 12 inches in a year. In many cases, a fish will be an adult in about one to two years.

What Factors Determine How Fast Fish Grow?

Growth is a characteristic of all things that are alive; humans, fish, mammals. The rate of growth not only varies from one species to another but sometimes in the same species. 

• Different localities — this can also cause the same species to differ in size. 
• Seasonal effects — this includes the different temperatures in different seasons. For example, fish grow faster in the summer than they do in the winter.
• Availability of oxygen and food — if there is not enough food in the area where they are, only the fittest will grow and survive. Everyone needs oxygen to survive so their area needs to provide them with enough oxygen in the water
• The density of population — research has shown that a lower density of fish will have a higher growth rate because there is enough food and oxygen to help them grow. When there is a higher density in the population, the growth rate will be slower.
• Abiotic factors — research has shown that fish found in the southern and northern waters will have a considerable variation in their growth rate. This is due in part to the temperature of the water as temperature plays an important part in how much food the fish eats and their metabolism. Light will also affect the pattern and rate of fish growth.

Various Fish: How Fast They Grow

• Arowana — this is a freshwater bony fish that many choose to put in their aquarium. It is a fast-growing fish and can grow two inches a month, which is almost two feet in a year. They can grow to as much as four feet long so that would mean they would reach full growth in approximately two years. 
• Guppy — this is also a popular fish for your aquarium fish and it reaches adulthood in about six months. 
• Trout — when they are born it will take them four to twelve weeks to become a fry, which is big enough to stock ponds, and then it will take about six months to reach a fingerling size of 8-10 cm and about a year to reach market size.
• Goldfish — they grow an average of one to four inches each year.
• Tilapia — if the water is the right temperature, from the mid to high 70’s, they will grow to adult size in six to seven months. It will take them longer if the water is cooler.
• Koi — during adolescence they will often double in size.
• Bluefin Tuna — in the Mediterranean and East Atlantic, Bluefin Tuna will take four to five years to reach maturity but in the West Atlantic, it will take about eight years.
• Redfish — this is a common name given to several species of fish but is generally applied to deep-sea rockfish or the reef-dwelling snapper. As juveniles, they can grow as much as an inch a month. As an adult, they tend to grow in girth and not inches. On average, by the time they reach five years of age, they are only grown two inches a year.
• Mahi Mahi — one was tracked and in 18 months, it went from 1.5 pounds to 68 pounds but their growth will vary.
• Crappie — after a period of two years they will grow 201mm to 275mm. When it gets to around 225mm, they will start to grow in poundage and not in length.

Fish and Growth

Fish exhibit what is called indeterminate growth, which means that throughout their life, most will continue to grow. As long as the conditions for growth are good, then they will continue to grow in weight and length. Although fish may stop growing, they will not necessarily die but if the resources that caused it to stop growing improve, then in the future, it may start to grow again. 

Also, when talking about growth, many think of it in lengths while growth can also mean pounds. It is all how you interpret the question. Catfish can grow up to two pounds in one year while the yellow and blue catfish can grow to full size in just six weeks and weigh 75-130 pounds.

Conclusion

On average, most fish will grow into their full size within two years but it all depends on their species. Some fish grow rapidly for the first two years and then start to slow down in their growth. And many species of fish do not stop growing but as they get older the rate of growth will slow down. Most of the growth of a fish is done when they are young, and it is usually at a rapid pace.

Some will gain weight but do not grow much in length. For example, a bass that weighs seventeen pound may go on an eating binge and will put on weight but not grow longer while another one of the same weight may not go on an eating binge and gain a couple of inches but no weight gain.

As mentioned, all fish grow at different speeds plus you need to look at all the factors. Remember, growth does not always mean inches, it can also mean pounds. If you are raising fish to sell, you want the rate of growth to be in pounds so you will make more money. After all is said and done, the most important factor that affects how fast grows is water temperature.

If you are raising aquarium fish, make sure that the fish will be comfortable in the size tank you have before you buy the fish. Ask about their rate of growth.