Michael Minner, age 13, of St. Louis, Missouri, for his question:
What do they mean by a balanced aquarium?
Space scientists are very interested in the operation of the everyday balanced aquarium. They are busy planning chemical and biological systems to support space travelers on their long, isolated voyages, and the balanced aquarium provides an example of such a self operated, self contained biological system.
An aquarium, of course, is any container designed to keep living fish. The water in an ordinary goldfish aquarium must be changed frequently and the bowl requires regular cleaning. Sometimes a constant supply of air must be pumped into the home aquarium. The sophisticated balanced aquarium is designed so that these housekeeping chores are taken care of within the tank itself. In theory, it should remain untouched by human hands indefinitely. Only a little fish food at regular intervals is required. In practice, however, its balance is rarely perfect, and once every few months or so it must be emptied and cleaned like any ordinary aquarium.
The balancing factors are borrowed from one of nature's own ideas. Fish require constant supplies of vital oxygen dissolved in their water. In the vast outdoor world, surface waters and tossing waves dissolve oxygen from the gaseous air. More oxygen is added by seaweeds and drifting algae plants. Fish use up oxygen and return waste carbon dioxide to the water, just as land animals use gaseous oxygen and return carbon dioxide to the air.
Both land and water plants use carbon dioxide in photosynthesis and return waste oxygen. A balanced aquarium borrows this two way system of give and take. It contains an assortment of living fish that use dissolved oxygen and return carbon dioxide to the water and an assortment of thriving aquatic plants performing the reverse process.
The fish and plants create a continuous system of exchange. Waste carbon dioxide from the fish becomes the vital gas for plant photosynthesis. The waste by product of photosynthesis becomes vital oxygen for the fish. The system aims at a balanced give and take to support the gaseous needs of an enclosed world of plant and animal life.
Sometimes, however, microscopic algae plants invade the aquarium, becloud the glass walls and befog the water with murky green haze. This problem can be solved by borrow¬ing another one of nature's ideas. In nature, water snails feed on single celled algae. So we add a few water snails and let them perform house cleaning chores as they crawl about their daily routines of algae eating.
Our balanced aquarium should be kept at comfortable room temperature near a window that sheds plenty of gentle, indirect sunlight. A sliding glass roof helps to keep out dust. The fish need a daily helping of just enough food for one meal. A little water will evaporate, so once in a while you add enough to bring the tank up to its usual level. Almost all the other aquarium duties are performed as routine operations by the balancing factors of the living fish, plants and snails.