The composition of sea water is as follows:—
| Water | 96·47 | per cent. |
| Sodium chloride | 2·70 | ” |
| Magnesium chloride | ·36 | ” |
| Magnesium sulphate (Epsom salts) | ·23 | ” |
| Calcium sulphate | ·14 | ” |
| Potassium chloride | ·07 | ” |
| Traces of other substances | ·03 | ” |
| 100·00 |
and it will be seen from this table that artificial sea water may be made by adding about three and a half pounds of sea salt, obtained from the sea by the simple process of evaporation, to every ninety-six and a half pounds of fresh water used. In making it there may be some difficulty in determining the weight of the large volume of water required to fill an aquarium of moderate dimensions, but this will probably disappear if it be remembered that one gallon of water weighs just ten pounds, and, therefore, one pint weighs twenty ounces.
If the sea salt cannot be readily obtained, the following mixture may be made, the different salts being purchased separately:—
| Water | 96½ | lbs. |
| Sodium chloride (common salt) | 43¼ | ozs. |
| Magnesium chloride | 5¾ | ” |
| Epsom salts | 3¾ | ” |
| Powdered gypsum (calcium sulphate) | 2¼ | ” |
Although in this mixture the substances contained in the sea in very small quantities have been entirely omitted, yet it will answer its purpose apparently as well as the artificial sea water prepared from the true sea salt, and may therefore be used whenever neither sea salt nor the natural sea water is procurable.
Assuming, now, that the aquarium has been filled with sea water, it remains to introduce the animal and vegetable life for which it is intended; and here it will be necessary to say something with regard to the amount of life that may be safely installed, and the main conditions that determine the proportion in which the animal and vegetable life should be present in order to insure the greatest success.
Concerning the first of these we must caution the reader against the common error of overcrowding the aquarium with animals. It must be remembered that almost all marine animals obtain the oxygen gas required for purposes of respiration from the air dissolved in the water. Now, atmospheric air is only very slightly soluble in water, and hence we can never have an abundant supply in the water of an aquarium at any one time. If a number of animals be placed in any ordinary indoor aquarium, they very soon use up the dissolved oxygen; and, if no means have been taken to replace the loss, the animals die, and their dead bodies soon begin to putrefy and saturate the water with the poisonous products of decomposition.
It is probably well known to the reader that a large proportion of the oxygen absorbed by the respiratory organs of animals is converted by combination of carbon into carbonic acid gas within their bodies, and that this gas is given back into the water where it dissolves, thus taking the place of the oxygen used in its formation.
If, then, an aquarium of any kind is to be a success, some means must be taken to keep the water constantly supplied with fresh oxygen quite as rapidly as it is consumed, and this can be done satisfactorily by the introduction of a proportionate quantity of suitable living weeds, providing there is not too much animal life present.