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How Big Will My Fish Get?

Fish Growth and Size in a Home or Office Aquarium

(The First Tank Guide)

It simply is not true that fish will only grow to fit their tank. This is an "old wives' tale" and one of the most common, and sometimes most frustrating, myths about fish keeping today. This misinformation is perpetuated by the fact that most fish will die before they visibly outgrow the tank they are housed in. This early death is unnecessary and can be easily avoided by providing enough space for the fish to reach full size, along with proper care for the fish and their fish tank.

How Big Will My Fish Get?

Almost all cold-blooded animals, including fish, will continue to grow as long as they are alive, and may outgrow a small tank. however, this continued growth is within reason and based on the normal template laid out by mother nature. The "full grown" size of a fish is determined by genetics, not by aquarium size, and individual fish will reach different sizes, just as adult humans reach different sizes.

Other factors can influence the size a fish will reach, just as they will in other animals. These factors include quality of diet, available nutrients and resources, cleanliness, volume of diet, and available space. Not only will these influence the mature size of the fish, but they will also influence the growth rate and overall health of the animal. A fish that does not have enough protein and vitamins in their diet will grow more slowly, stay smaller, and be less active, however, this fish will also be more prone to injury, heal more slowly, be more susceptible to disease, be less able to tolerate aggression or abuse from other fish, be more sensitive to water quality issues, have a higher risk or organ failure, be less likely to spawn successfully, and be generally weaker. The same is true for insufficient space or poor water quality. Additionally, the decrease in overall health of these fish is far greater than the small change in their adult size.

What Other Problems Are Related to Small Tank Size?

Other health problems caused by keeping a fish in a tank that is too small may include skeletal deformities, digestive problems, and weakening of the skin and scales. Some of these problems are caused by different changes in the rates of growth of different tissues in the fish due to the elevated toxin levels in the water in an overpopulated tank. All fish will have their immune systems repressed when they are kept in a tank that is too small or too crowded. This results in more frequent outbreaks of disease, as the fish are less able to resist pathogens. This also makes it more difficult to treat diseases when they do occur, as you do not have the help of the fishes' own immune systems to help fight off the disease.

Keeping fish in an undersized tank can also result in suppressed appetite. This will further complicate things. First, if the fish are eating less, it is harder to not overfeed the tank. Overfeeding the tank will result in a further degradation of the water quality, compounding the vicious cycle of tank overpopulation. Second, fish with a suppressed appetite are less likely to be providing themselves with the nutrients that they need to grow and remain healthy. Though this will keep the fish from growing as fast, and it will reduce their chances of reaching their full adult size, it also further complicates the problem by making the fish less healthy, and therefore less able to resist the stresses of being in a tank that is too small.

The poor water quality a fish experiences when it is kept in an aquarium that is too small can also cause a weakening of the gills, kidneys, liver, and/or heart. This happens because the blood cannot be efficiently cleaned by the gills and the kidneys, when the water that the fish is constantly absorbing has a high level of waste in it. The fish normally would eliminate this waste through the gills and anus, however, in a tank that is too small, the waste cannot be dissipated in the water, so the fish reabsorbs it as he swims.

"Fantastic web site you have created. Loads of useful info."
December 31, 2003
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"Although I've had a successful tank before, there are always things to learn to ensure another happy, healthy community. I'm thrilled that I found your site. I have bookmarked it and will be returning to it, many times I'm sure, when everything is in place to begin again. I like the way you present things. You put it out there...if you [want] a successful tank, there are certain things that need to be done."
March 10, 2019
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