Only the area of water surface is pertinent in figuring the capacity of a pool or aquarium for fish; depth does not count at all. Goldfish in a 10- by 12-inch aquarium will be quite as well off in a 1-foot depth as in a depth of 3 or 4 feet.

Water gardeners, applying the capacity rule, often inquire whether

They should count the entire water surface or deduct the area covered by water-lily leaves and other foliage. Do not deduct. It is true that foliage does cover a great deal of water surface in outdoor pools, but it does not seem to affect the surface absorption of oxygen. Turn over a floating water-lily pad, and you usually find the underside covered by air bubbles, bubbles released by water when it had an overabundance of oxygen. It stands to reason that these bubbles will be reabsorbed if and when the oxygen content of the water lowers a little.

The beneficial effects of oxygenating plants are not to be considered when figuring capacities. Their healthful influence should be provided in addition to plenty of living space. Also remember that oxygenating plants do their good work only under the influence of light. At night, unless very strong artificial illumination is provided, pool and aquarium water must have the oxygen content replenished entirely by surface absorption.