Is it necessary for a tropical fish tank to get daylight?

It is not necessary for an aquarium tank to get daylight. As a matter of fact, daylight, being difficult to control, frequently makes it difficult to balance a tank properly. A location well away from the light is perfectly satisfactory. An aquarium frequently serves a dual purpose as a lamp in a dark vestibule or corner. Should it be desirable to keep an aquarium near a window, do not be deterred by the foregoing, since there are ways of controlling daylight to make it suit your purpose. A northern exposure is best, though for second choice a western exposure is preferable for a tank located near a window.

For all practical purposes, sunlight is unnecessary to an aquarium. Not only is it unnecessary, but it can actually be dangerous and harmful if the aquarium is exposed to it to excess. The difficulty with sunlight is that it is hard to control, and its occurrence is impossible to predict. A little sunlight for an aquarium situated in a window may be beneficial during the colder months when the rays are weak. Those same rays may become a menace during the warmer months of the year, even cooking the fishes and plants in extreme cases. What usually happens is that the best heat of the sun warms the aquarium considerably during the day. At night there is a sharp drop in temperature. Most tropical fishes are mildly tolerant of a minor temperature change which does not occur too often. When it is both sharp and frequent, it usually leads to complications and disease.

Besides the heat problem, the strong rays of direct sunlight frequently trigger the growth of microscopic plants known as algae.

Daylight or indirect sunlight, being so much less intense than direct sunlight, is easier to control and more beneficial. Natural light must be taken into account when computing the length of time the reflector should be kept lighted. The more natural light a tank receives the less artificial light it requires, and vice versa.

Aquarium Algae

Some algae are small single cells and form scum on rocks; others, such as various seaweeds, are very large.

The spores of many of them are extremely hardy, can resist drying and freezing, and may be air-borne. A completely sterile aquarium, unless it is sealed shut, can develop the algal growth from spores deposited in it through the air. A pond that dries up every year develops algae when the rains refill it and the sun warms and lights it.

The most common form found in the aquarium is the fuzzy algae that forms on the glass under the reflector, or wherever light strikes the tank. More objectionable are the slime algae that form slick sheets on the glass, rocks, and plants. Most dangerous are the bright blue-green algae that may be poisonous.

Not so serious, but quite annoying, are the thread algae which blanket the bottom and rocks. Then there is “green water,” which is simply an uncontrolled growth of free- floating algae. In extreme cases this can become so thick that aquarium inhabitants only an inch away from the glass are invisible.

Many aquarists cultivate aquaria of “green water,” believing that it has great therapeutic value. Many cures are attributed to the simple art of placing a fish in a tank of “green water” for several weeks. Nevertheless, “green water” of all Algae is potentially the most dangerous to fishes.

During the warmer months the oxygen content of the aquarium is at its lowest. Should the algae causing the “green water” be deprived of light, even for only a few hours, they start a chain reaction in which the algae use up all the available oxygen in respiration. Deprived of oxygen, the algae start to die. Normally, the lower areas are first affected, as they are farthest from the surface where oxygen enters. Being dead, the tiny plants decompose rapidly in the warm water. Decomposition also uses up oxygen and gives off carbon dioxide. This hastens the process, and a tank of “green water” can thus become an odorous mass of rotting algae within a few hours.