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Tuatara: Volume 5, Issue 1, January 1953

Principles of the Balanced Freshwater Aquarium

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Principles of the Balanced Freshwater Aquarium

Although the freshwaters do not offer such exotic fauna as the seas, when properly established the freshwater aquarium is a valuable asset in the classroom and an object of beauty in the home. The variety of freshwater plants amenable to aquarium cultivation permits the setting-up of a permanent aquarium carrying many useful and attractive animals and plants. It facilitates the study of life-histories and demonstrates many important biological principles.

The common goldfish bowl—a bowl placed in the sun, with two fish but no gravel or plants—illustrates nearly every improper practice in tank management. Fed every day, the uneaten food and droppings from the fish are swirled up as the fish swim. Bacteria and protozoa bloom and cloud the water depleting it of oxygen, which in any case is in short supply because of the restricted surface area relative to the volume and the low solubility of the gas in warmed water. The fish gulp air at the surface. The water is changed, sometimes daily, to ‘keep it fresh’. A goldfish bowl proves one thing, goldfish are hardy creatures; but few other animals have this measure of hardiness and require instead the delicately adjusted environment of a ‘balanced’ tank. This is a tank so organised that the animals will enjoy as near stable an environment as possible. The equipment is a tank; clean water; a bottom of coarse sand or fine gravel; plants; animals; a cover; control of illumination; oxygen; and food. These items are combined with due regard to the influence of one factor on the other, to produce the ‘balanced’ tank, thus providing the highest measure of environmental stability which with experience can be adjusted to suit the need of one animal or another, or many.

All ‘balanced’ freshwater tanks are static. Aeration and circulation are not needed. The tank should be as large as possible. Ten gallons is a very small tank, twenty-five gallons a good size to start with. Shape is important. The width should be equal to, and the length double at least of the depth to provide a surface area adequate for oxygenation of the water. The frame should be sturdy and of metal with the bottom, sides and ends filled in with glass—never wood or metal. Slate and marble make an excellent but heavy base and ends. Wood waterlogs or warps, metal rusts unless painted and page 2 too many paints are toxic. The glass should be mounted in the frame using a non-hardening putty, never an oily putty for the oil will bleed into the tank, float on the surface and so prevent gas exchange. A hardening putty may hold its seal on the glass for years, or break on the least torsion of the frame. To repair the seal is most difficult, and reglazing is preferable to patching but often means broken glass for it is not easy to free the glass from old putty.

Before filling a tank, take these steps: (1) Check the seal. The putty should be firmly and cleanly in contact with the glass, air-bubbles and other breaks can mean early leaks. (2) Select a permanent location for the tank since a tank must be emptied before moving, and this can throw a tank out of ‘balance’ for some days. Place the tank where no direct sunlight can strike it. If near a window, have one end towards the window. (It can be placed closer to the window in a south-facing room than in a north-facing room.) (3) Place the tank on a base which adequately supports the frame so that there is no torsion as the tank is filled.

Wash the tank out with water. Fill to an inch from the top preferably with clear pondwater but clear tapwater will do. Cover the tank with sheet glass to prevent loss of water by evaporation. Be patient. Leave the tank stand for two weeks to a month to ‘age’ the water.

Select material for the bottom of the tank with care. Soil, clay and similar material discolour the water or produce colloidal suspensions which will never settle. Obtain fine gravel or coarse sand with particles ranging in size from a pin-head to a match-head (white is more attractive than darker gravel or sand). Granules of this size permit debris and droppings from the animals to sink below the surface of the gravel where bacterial action is rapid, and do not clog the roots of the plants. Obtain enough to form a layer 1½ to 2 inches or more deep, and wash throughly with running water, stirring until the water runs away quite clean. Leave it stand several days covered with water, and wash again. Now added to the tank there should be little clouding and no discolouration. Spread evenly and then bank deeper towards the illuminated end of the tank and to the back, with a gentle slope to the front of the tank. If you plan to keep fish, make the bottom shelve to a wide, shallow, central hollow where uneaten food and faeces will collect and can be easily removed, or make a one inch deep groove for this purpose along the foot of the front glass.

Vallisneria is easily obtained, and a most useful plant. Wild plants are best from ponds, not running water; but ‘tank-hardened’ plants are preferable and can usually be purchased. Plant an inch from the glass, crowded at the illuminated end and scantier along the back of the tank so that the greater part of the tank will be shaded by plants from excess illumination. Spread the roots deeply so that the plant is secure, but do not cover the crown. Elodea, Myriophyllum, Cabomba, Sagittaria, Aponogeton, etc., can be planted sparsely in open spaces as ornamentals. page 3 The plants have two functions in a balanced tank. First, they absorb plant foods formed in the decay of droppings and debris and so maintain the organic content of the water at a low level which holds back bacterial and algal blooms. Secondly, growing in response to available illumination, these plants spread into the more strongly lighted areas and so diminish the light in the rest of the tank which further hinders blooms of plants such as thread algae. Floating plants, like Lemna, can be used to control light entering through the top of the tank. Plants absorb oxygen twenty-four hours in the day, yield surplus oxygen during daylight but rob the water of this gas at night so that an adequate surface area is the only assurance of an oxygen supply for plants and animals alike.
Diagram showing the method of setting-up a tank with the major incidence of illumination (A) at the end; a dense planting (B) of aquatic plants such as Vallisneria used to reduce the intensity of illumination throughout the tank generally; a settling hollow (C); and ornamental plants (D) arranged for display in the open end of the tank.

Diagram showing the method of setting-up a tank with the major incidence of illumination (A) at the end; a dense planting (B) of aquatic plants such as Vallisneria used to reduce the intensity of illumination throughout the tank generally; a settling hollow (C); and ornamental plants (D) arranged for display in the open end of the tank.

Before stocking the tank, let it stand until clear, a matter of a week or more. This and the previous ‘aging’ period both give time for bacterial and protozoal blooms to take place. Such blooms cloud the water, often deplete the oxygen supply; but as they pass, leave the water crystal clear, as it will remain for even twenty years if the tank is properly managed.

In stocking a tank with animals, a step is taken which determines the type of tank management. The usual set-up is a tank containing animals such as small fish. The old rule, and still a good rule used with common sense, is one inch of fish to a gallon of water—no more than that, and page 4 preferably less. Fish and other ‘large’ animals, e.g. crayfish, shrimp, etc., require feeding. This means the constant addition of organic matter to the tank. Some is converted to animal protein and so ‘bound down’ and inaccessible to the tank generally; but much is passed in excretions. In the properly set-up tank most of the droppings and uneaten food can be removed from the settling hollow in the tank. The rest collects in amongst the plants or between the plants and the glass where it is not swirled up by the fish, and undergoes decomposition. The break-down products feed the plants which after a year or so can be thinned if needed. If algal blooms appear early in the life of a tank this shows that the tank is under-planted. More large plants must be added to absorb surplus plant nutriment and reduce illumination. If plants die down or fail to grow more illumination may be needed. Too little light and stunting of plants will lead to an accumulation of dissolved organic material, blooms of bacteria and explosive blooms of diatoms and algae if the illumination increases. The ‘balance’ with plants is that they must not deplete the tank of oxygen during the night; they must not overcrowd the tank so that the animals cannot be seen; and they must prevent algal and other blooms by utilising all available nutriment and controlling illumination.

Alternatively, an aquarium can be stocked with minute animals which do not require feeding. This is the ideal tank for the classroom for it can contain protozoa of many kinds, Hydra, turbellarians and other aquatic worms, rotifers, Daphnia and other Cladocera, Ostracoda, Copepoda, Amphipoda, aquatic insects and aquatic insect larvae, the smaller aquatic snails—in fact a wide variety of small life which can be kept available for study at any time. Moreover, since growth is slow, it will hold filamentous algae as well as the larger plants. The tank is set up essentially as before and ‘seeded’ with the washings from wild aquatic plants, debris from the floor of a pond, scrapings from rocks and logs submerged in streams and pools, etc. Only small quantities are required for seeding. Too much will foul the water. Avoid carnivorous animals such as beetle larvae, diving beetles, etc., and above all things avoid filter-feeding animals such as freshwater clams. (Since freshwater clams and mussels can individually filter up to twenty gallons of water a day, if introduced into a tank they quickly destroy the cycle of micro-organisms.) Out of the seedings will develop a community of small animals none of which require feeding and so the organic content of the tank remains constant.

Balancing this type of tank takes time, five months or more. The aim is to produce small continuous crops of floating bacteria, unicellular plants, and protozoa for minute filter-feeders such as Daphnia etc. which in their droppings and from their dead bodies will provide fertiliser to continue the cycle, and by their feeding prevent excessive blooms (green water) of unicellular algae. Accordingly, larger aquatic plants are required to a less extent in the first planting of a tank of this type (use about one crown of Vallisneria to every two gallons of water). Growth of these plants will be page 5 much slower than in a ‘fish’ tank. Nitella grows rapidly and is useful in the early years of these tanks because it flourishes on available nutriments but will die down quickly when starved. The quickest and simplest control of bloom organisms comes through successful seeding with pond cladocerans such as Daphnia which can be obtained in random collections. Any hint of ‘green water’ calls for more large plants and fresh seeding of the tank. Throughout the life of the tank it is sound practice to reseed at least once or twice in the year.

The theory of ‘balance’ outlined above is rule-of-thumb, but well-tested in practice. It centres on the control of the organic content of the water and of illumination by the use of larger aquatic plants so preventing blooms of microscopic plants and algae which cloud the water, fog the glass, and ‘spoil’ the tank. In general such blooms rob the water of oxygen, and some at least produce highly toxic substances such as saponins. The theory is concerned largely with the organic nitrogen cycle and takes little account of carbohydrates which are usually regarded as being balanced out by oxidation to carbon dioxide and water; but many of the breakdown products from higher carbohydrates are far too stable to be eliminated from the tank in such a simple manner and these probably increase in concentration over the years. However, this is disregarded in ordinary management but is still worth investigation. The theory recognises the importance of essential minerals and some aquarists consider that the binding-down of these substances by the larger aquatic plants is the primary factor in prohibiting blooms of algae and diatoms; but the case is not proven, and in practice all that is required is to regard the larger aquatic plants as controllable agents which permit control of the lesser elements in the tank and so bring the tank to ‘balance’.

This theory provides the answers to the many questions which are so often asked. Are snails needed to keep the glass clean? Are clams needed to filter the water? How often should the water be changed? In each case, the answer is the same—never, with a properly balanced tank, for there will be nothing to cloud the glass, fog or foul the water. Simple consideration of the theory provides the answers to all the problems which arise and places tank management on an understandable basis.