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The aquarium

  • As a general rule for estimating the size of the tank needed, allow 4.5 litres of water for every 1 centimetre length of fish (including the tail). A good starting size is 60 centimetres × 45 centimetres × 40 centimetres high.
  • The tank should have enough water surface for oxygen to be absorbed efficiently from the air, and it should have a deep front for viewing. Its carrying capacity is influenced by the size of the fish, the size and amount of tank furniture and hiding places for fish, feeding regimes, whether the tank is filtered and aerated, the water temperature, and waste metabolites present in the water. Furniture, plants, filtration, and aeration all increase the carrying capacity.
  • A cover is essential to protect the aquarium from dust, pollutants, and predators. It will also help to maintain an even water temperature.
  • The tank should also have an aeration pump, an under-gravel filter, river gravel, and plants. Plastic plants are easier to care for than live plants.

Setting up the aquarium

  • Prepare the aquarium one or two days before placing the fish into it so that the chlorine in the water will evaporate. Alternatively, use chemical products that achieve this more quickly.
  • Prepare the tank by washing it out with salt and water and rinsing it thoroughly.
  • Place the empty tank on a flat, solid surface out of direct sunlight. As 1 litre of water weighs 1 kilogram, when the tank is filled with water and gravel, it may be very heavy. Consider what could happen in the event of an earthquake.
  • Gravel is to a tank what soil is to a garden, and so there should be a generous depth of about 5–10 centimetres. Before placing gravel in the aquarium, rinse it under a stream of water until the water runs clear. Fill the tank about one-third full with water and follow instructions for the installation of an under-gravel filter.
  • Plants not only provide hiding places for the fish, they also make the tank look nice. Real plants assist the biological filter in reducing ammonia levels, and they appear to reduce algal growth, although most of this is an aesthetic problem only.
  • Plants provide small amounts of natural food. Sometimes goldfish will eat them faster than the plants can grow. When placing the plants, put the tallest at the back and the shortest in the front.
  • Continue to fill the tank to within 5 centimetres of the top and turn on the pump to check that all systems are working.
  • Do not release the fish straight into the tank because the sudden temperature change may kill them. If the fish are in a plastic bag, put the unopened bag into the tank water for about an hour to adjust the water temperature before releasing them.

Respiratory gas exchange

  • Fish need oxygen to breathe. They breathe through their gills and extract oxygen that is dissolved in the water. Oxygen diffuses readily into water, but the water temperature will affect the amount that can be dissolved. Cold water holds more oxygen than warm water.
  • Sources of oxygen in the aquarium include diffusion at the surface of the water and an aeration system. Do not rely on plants alone to provide oxygen because, although they photosynthesise during the day, they use up oxygen at night. Fish, plants and phytoplankton (at night), decaying organic matter, and bacteria all use up oxygen.

Maintaining the aquarium

  • How often the water needs changing depends on the number of fish and the effectiveness of the filtration and aeration systems. As a rule, remove and replace one-third of the water every two weeks. Remember to let replacement water stand in a bucket for twenty-four hours to dissipate the chlorine before adding it to the aquarium.
  • Plastic plants will develop an algal growth. Wash them in warm water and leave them to soak in a weak bleach solution (10–20 millilitres of bleach per litre water) for ten to fifteen minutes. Rinse them thoroughly before returning them to the tank.


  • Filtering the water removes waste products and helps to maintain good water quality and good fish health. There are two main kinds of filtration – mechanical and biological. Chemical filtration is rarely used.
  • Mechanical filtration removes material suspended in the water by pumping it through a filter material, such as glass wool. Clean this filter every one to two weeks, depending on the size of the tank and the number of fish it contains. Even when the water looks clean and clear, it can still contain dissolved wastes that may affect the fish.
  • Biological filtration basically relies on the work of beneficial nitrifying bacteria that colonise the surface of the gravel and rocks in the aquarium. Under-gravel filters allow the whole gravel bed to become colonised. As the gravel may act as a mechanical filter as well, siphon off debris sitting on the surface of the gravel once a week. One-third of the gravel should be removed every eight to twelve months and rinsed lightly in water to remove accumulated debris. Don't wash all the gravel at once because you may lose the 'good' nitrifying bacteria.

Aquarium diagram

Goldfish aquarium: main features

  • Remove and replace one-third water every two weeks.
  • Allow chlorine to evaporate when refilling tank after cleaning.

Aquarium diagram explanation

The glass tank 60cm x 45cm x 40cm high shown is a good starting size. It must have a cover to keep out predators and be kept out of direct sunlight.

  • A small square aeration system is shown outside the tank with two tubes leading into the tank – ventilation is important.
  • A small filter pump is shown leading into the tank to a tube that needs regular filter changes.
  • Inside the tank there is washed gravel on the tank floor and rocks and plants to provide shelter and assist filtration.
  • Every two weeks one third of the water should be removed and replaced.
  • When refilling the tank after cleaning, the chlorine needs time to evaporate.
  • The jar of fish food outside the tank is a reminder that the fish need a daily feeding with commercial fish food.

How to care for goldfish


  • Feed the fish once a day, using commercially available goldfish flakes or granules.
  • Feed only amounts that can be eaten within a few minutes because uneaten food decomposes on the bottom, increasing the ammonia levels in the tank.
  • Remember to make arrangements for feeding outside school and ECE centre hours and during weekends and holidays.


  • Learn what is normal behaviour for fish and check them daily.
  • Fish can and do get sick, often because of their environmental conditions. Consult a veterinarian about this.
  • Beware of ammonia build-up, which is toxic to fish. Ammonia is the main metabolic waste product of fish, and it also comes from decomposing food and other organic material. Increased levels of ammonia can also be caused by overcrowding, overfeeding, high water pH, warm water, and new tanks that haven't had time to develop good populations of nitrifying bacteria. Ammonia levels can be controlled with a good water flow, regular water changes, and not overfeeding the fish.

Interesting facts about goldfish

  • Goldfish (cyprinids) are cold-water, herbivorous fish. They are poikilothermic (cold-blooded), and so their body temperature is determined by their environment.
  • Goldfish are egg layers and provide no parental care. They usually deposit their eggs on vegetation or near the surface of the water.


The text and illustrations for this online edition of Caring for animals: A Guide for teachers, early childhood educators, and students (published on Te Kete Ipurangi for the New Zealand Ministry of Education in 2005) is copyright © Crown 2005. All rights reserved.

Content has been adapted for the web from the printed version, originally published in 1999 by Learning Media Limited for the Ministry of Education. Although no longer available this publication may still be available in some schools.