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Fish only grow to the size of the tank

Question: Is it true Fish only grow to the size of the tank?

Answer: Yes..! But only because they’re DEAD

I hear this day and daily, and it is total and utter RUBBISH. It’s true that fish never outgrow their tank, but only because they die prematurely.

Many animals, ourselves included,  grow to about 80% of their adult size in the first 10% of their life, and while some stop growing having reached adult size, others such as fish continue to grow, albeit more slowly, until death.  It is well known that poor living conditions and malnutrition can stunt growth and cause illness and deformities in any animal but that’s not really the question being asked.

This old wives tale to which we refer, suggests that if we keep a fish, one that would normally grow quite big, in a small aquarium it will stop growing before it outgrows the tank, but continue to live a normal and healthy life. (If you believe that can I recommend a great investment, I’am starting a company to mine cheese on the moon and sell it to Tesco’s at a vast profit. If you get in now you could make a fortune!)

In reality if we buy a potentially big fish, an Oscar for example, as a 3” juvenile and place it in a 60L aquarium it will for a time continue to grow at its normal rate, which, for an Oscar with a life expectancy of 20 years and an adult size of 12” – 14” would mean another 7” in the next 2 years.

It is simply not going to happen. As the Oscar grows we would be forced to increase its food supply, or not and simply let it starve to death. If we do increase its food to match its body size, then its waste production will also increase accordingly, and there is only so much waste which can be accommodated in a 60L aquarium, no matter what size your filter is! As the increased waste production turns the water toxic the fish’s organs begin to fail and it dies. Either way, it never gets to outgrow the tank.

Always consider the potential adult size of the fish that you purchase, as well as their life expectancy. For many this may be only 1” – 2”, and 1 – 2 years, but for others it can be considerably more. At Grosvenor Tropicals it’s our policy not to stock the so-called “Tank Busters”, but the term does depend somewhat on the size of your aquarium. Whilst at home in a 500L aquarium, an adult Oscar would be considered a “Tank Buster” in anything  smaller!


Maroon Clownfish in an Anemone