The Effects of Keeping Fish in Too Small of a Tank

Home aquarists have two different ways to find themselves with a tank that is too small for their fish: there may be too many fish for the tank, or the species of fish may be too big for the tank. While tank size alone is not the only parameter to consider (factors like filtration are important, too), too-small tanks are a known cause of illness and death in fish.

  1. Poor Water Quality

    • Fish give off waste in the form of ammonia, which is broken down through the nitrogen cycle into nitrite and then nitrate. Ammonia and nitrite are toxic to fish and, in high concentrates, so is nitrate. All three contribute to the overgrowth of algae. In an aquarium that is too small for its fish, the volume of water available to dilute these waste products is smaller, so their concentrations are higher. This is especially true for nitrate, which continues to build until removed either by live plants or water changes. Filtration does not remove nitrates. Fish whose tanks are too small may also suffocate as they use up all the available oxygen. Symptoms of low oxygen, which is especially common in tall, narrow tanks with low water surface area, include fish gasping near the surface and death.

    Aggression

    • Many fish species, particularly those described as "semi-agressive" or "aggressive," are territorial. With less territory to go around, aggression among these fish tends to increase. This leads to chasing, biting and other forms of aggression, and often the weakest fish in the tank becomes sick or emaciated and dies.

    Stress and Disease

    • Poor water quality, increased numbers of water changes (which may be needed to maintain an overcrowded tank's water quality) and territorial pressure all increase stress on fish. The results of excessive stress range from poor growth and coloration to increased susceptibility to diseases like ick and fungus. Stressed fish are less able to recover from shocks such as a power outage, a failed heater or even a large water change. Either a shock like one of these, or the cumulative effects of stress, can cause premature death. Also, stressed fish rarely reproduce in an aquarium.

    Stunting

    • It is a common myth that fish kept in too small of a tank will "grow to fit the tank." A large species will generally continue to grow until it reaches its adult size, regardless of the size of its tank. In extreme cases, a fish without adequate room to turn around may be stunted or deformed by trying to swim in a small space. These fish will also be unable to engage in natural behaviors. In fact, keeping a large species in too small of a tank is a form of animal cruelty.