278 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
without any of them and with the plants growing in little pots filled 
with sand or pebbles. These may be made to look like rock by cover- 
ing ordinary small earthen pots with Portland cement, or by cement- 
ing together pieces of stone. 
The stereoptyped question, u How many fish can I keep in my aqua- 
rium ” is simply offering temptation to the cupidity of the dealers, most 
of whom will sell you all the fish they can without regard to the con- 
sequences to you. In fact, with most of them the governing principle 
of their business seems to be that the greater the mortality of fish the 
greater the sale of them. The comparatively few aquaria in use, as 
well as hosts of abandoned ones, attest the short-sightedness of such a 
policy. 
There can be no rule by which to determine the amount of animal 
life that can live comfortably in a given quantity of water, it being a 
question involving the size of the fish, the amount of light (upon which 
the activity of the plant life depends), and more than all of temperature, 
which is exceedingly variable. For every 16 degrees of low T er temper- 
ature water will take up and hold in suspension about double the vol- 
ume (approximately) of atmospheric air or of oxygen liberated by the 
plants. Thus it may be inferred that water at 50 degrees will support 
double the number of fish that could live in the same quantity at 66 
degrees. When water is heated on a stove the air will be seen to leave 
it in bubbles as the temperature rises until all is expelled. After water 
has been boiled fish can not live in it at all until it is again charged 
with air by the use of a bellows or pouring it from one vessel into an- 
other, thus imitating the revivifying influences of nature in the dash- 
ing of the waves or in tumbling it over rocks. Water from which the 
air has been expelled will again recover its normal proportion of air 
by absorption alone, but the process will be slower. Good judgment, 
then, would indicate, in view of the widely-varying conditions to which 
aquaria are subject, that the path of safety would lie in stocking them 
to the minimum number of fish only. The question then naturally 
arises, u How can I know the minimum number?” This is a question 
which can only be determined by observation, and the only safe plan is 
to add a few fish at a time. If you should pass the safe limit, there 
will soon be a lack of air or oxygen in the water, and the fish will keep 
at the surface, with their mouths out sucking in air, or, in other words, 
getting their supply of oxygen from the atmosphere itself. When this 
occurs it will be necessary to diminish the number of fish or to change 
the water in part at least. The aquarium will be affected unfavorably 
by a change from colder to warmer weather, as a portion of the air will 
be expelled. Also by cloudy weather, as the plants lacking the stimu- 
lus of strong light will not liberate a sufficient supply of oxygen. If 
the animal life in the aquarium is limited to what can live in it com- 
fortably under the most unfavorable conditions, it must then be uni- 
formly successful. 
