For many reasons the best aquarium is a pond in the garden, for here we may have greater variety of animal and vegetable life, and beautiful surroundings of plant life too. The pond should be about three feet deep, and the banks should slope, so that there will be a little spade work at first. The bottom may be of clay, but it is better for many reasons to have cement. The points mentioned with regard to the indoor aquarium apply here also, but many of the arrangements are more easily carried out in the pond than in the tank.
Plants for the Aquarium.—The duckweeds float and need no planting. They spread rapidly. There are four kinds: the Lesser Duckweed (Lemna minor); the Ivy-leaved Duckweed (Lemna trisulca). Then there is the American Pondweed (Anacharis alsinastrum), a weed that almost blocks slow moving rivers and canals. It will grow either attached to the bottom or floating. Not only does it supply the fish with air but with food also. The Hornwort (Ceratophyllum demersum) needs no soil, and gives off much oxygen, though probably not so much as Vallisneria spiralis, a great favourite in the aquarium. Easier to obtain is the Water Crowfoot (Ranunculus aquatilis), and we may mention also the Broad-leaved Pondweed (Potamogeton natans), Close-leaved Pondweed (Potamogeton densus), the Perfoliate Pondweed (P. perfoliatus), the Curled Pondweed (P. crispus), the Starwort (Callitriche verna), the Grassy Pondweed (Potamogeton gramineus), and the enterprising boy will find many others for himself in ponds, streams, and canals. Foreign plants are sold by dealers. For the garden pond the beautiful water lilies may be obtained. The weeds should be prevented from occupying too much space, and if the creatures in the aquarium do not keep them in check by eating them it will be necessary to remove some of the plants occasionally.
Sanitation.—If a fish dies remove it at once, or its dead body will pollute the water. To clean a glass aquarium let the water run away through a siphon until only a few inches remain, then clean the sides with a piece of rag tied upon a stick. Now siphon the remaining water away, at the same time supplying fresh water. Do not throw a lot of food to the fishes. What they do not eat decays and poisons the water. If there are molluscs in the aquarium this danger is lessened, for many of them act as scavengers, and they are assisted by freshwater shrimps, tadpoles, and beetles. The shrimps, however, may eat the living as well as the dead, and the tadpoles, instead of being allowed to eat, may themselves be eaten.
The Food Supply.—Fish eat the buds and tender shoots of pond weeds. They may have also a moderate supply of small worms, gentles, different kinds of larvæ, and what are called ants’ eggs. Frogs and toads eat insects, little beetles, worms, grubs, caterpillars, and newts need an occasional worm.
The Fish.—Many of the fish that swarm in most ponds and streams are suitable for the aquarium. The carp is related to the goldfish, which is the golden carp. It is quiet and harmless, and will not interfere with other creatures in the aquarium. Minnows are pretty, and should have running water, and the roach is another suitable fish for life in captivity. The gudgeon, loach, and bullhead serve for bottom fish. Jack and sticklebacks are extremely interesting, but need a place for themselves, as they eat any other inhabitants of the same aquarium.
Fishes as Pets.—Perhaps, writes one of our contributors, the most interesting of freshwater fishes to watch is the stickleback, especially if a pair can be kept during the breeding season in a good-sized aquarium, so that they have the opportunity to collect materials and build their curious nest in the natural way.
The male fish develops during this season a most beautiful vermilion-coloured breast, and is exceedingly pugnacious, so that it is useless to attempt to keep two males in the same aquarium.
In fact, the stickleback is often so quarrelsome that it is not desirable to try to keep other fish with him, unless they are much larger than he is.
The easiest fish to keep alive are the carp (to which tribe the goldfish belongs) and the minnow.
These can be kept for years without much difficulty, and will not be likely to injure one another, or any other fish that may be placed in the same tank.