If you spend enough money at the pet store, you can build an aquarium that will let you keep and display the fanciest, most colorful tropical fish anyone could want. As long as the temperature is controlled, the water aerated with a pump and the enclosure kept scrupulously clean, your fish will thrive and put on a fascinating show. But what you have is not nature, it's a cage for pet fish.
A cheap aquarium is a natural ecosystem, tiny but quite real. Start with the glass tank from the pet store, but fill it with plain water and some ordinary mud from a nearby pond. After it sits in the sun for a few days, put in a few young plants taken from the same pond, and wait until they begin to grow, and algae (another useful source of oxygen in the water) begins to grow on the tank walls. Then you're ready to introduce a very few small fish and tiny water animals, which you get from the same pond by scooping with a home-made net made from coat-hanger wire and an old nylon stocking.
If you can resist the temptation to add too many fish, your little aquarium will have a balanced oxygen/carbon dioxide cycle. The plants breathe in (and, in a sense, eat) the carbon dioxide in the water and breathe out oxygen. The fish, glassy little arthropods and bacteria breathe in oxygen and breathe out carbon dioxide. All you add is a little food. This aquarium needs no bubbler, and the only cleaning you should have to do is to scrape the front glass so you can see in.
In an ordinary aquarium, you're seeing fish. But in a cheap aquarium like this, you're seeing nature, exactly as it really is in the pond outside. It is a different but very inspiring experience.
(By Charles Ott. Reference: King Solomon's Ring by Konrad Lorenz,
1952.)
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