BULLETIN OF THE UNITED STATES FISH COMMISSION. 279 
Fish should not be introduced into the aquarium until at least a week 
after the plants are placed therein — except perhaps a comparatively 
small number to interest the children who are generally extremely im- 
patient to have them — so that the plants may first begin to grow and 
give off oxygen freely. Others may then be introduced from time to 
time until the aquarium is fully stocked. 
With regard to light it may be said confidently as a result of ex- 
tensive observation and experiment, that it is almost impossible to get 
sufficient from a northern exposure, the exception being in bay windows 
which may catch the morning or afternoon sun for a short time and 
have a good light generally. 
Professor Huxley goes so far as to say that “ plants only liberate 
oxygen under the action of strong sunlight, and in the dark this action 
is reversed.” This would account for the sometimes unaccountable 
deaths of fish in aquaria, which generally occur at night, when in addi- 
tion to the lack of light there is less ventilation and greater escape of 
poisonous gases from gas-pipes and furnaces. These gases are rapidly 
absorbed by water, and when the conditions are such that there are no 
counteracting influence their presence must be baleful in the extreme. 
In winter the full power of the sunlight may be allowed, but in the 
summer when the rays have greater heating power some modification 
is necessary, and the proper mean of sunshine may be found by a little 
experimentation. 
In rooms where there is a high temperature a close-fitting glass cover 
to the aquarium will be found beneficial. This advice invariably in- 
duces the question, “Will not that exclude the air % ” The answer is, 
that in a properly conditioned aquarium the water does not derive any 
benefit from contact with the more or less impure air of ordinary living 
rooms at all in proportion to the detrimental influences, but depends 
wholly upon the plant life for its supply of oxygen. The glass cover 
prevents the absorption of impurities from the atmosphere and the col- 
lection of dust, and by retaining the oxygen which is given off in excess 
of what the water will hold in suspension there is always a highly oxy- 
genized atmosphere immediately over the water. 
Another and very common question is, “ How often should I change 
the water in my aquarium ? w A successful aquarium is one which is so 
stocked and afforded theproper conditions, in accordance with the princi- 
ples already given, that it will not be necessary to change the water- 
other than the change effected by supplying the loss from evaporation— 
except at long intervals, not more than once or twice a year when a 
general cleansing may be considered desirable. 
There are many experienced aquaria fanciers who find no difficulty in 
keeping (both marine and fresh water) aquaria for many years without 
change of water. The water should not be changed unless the fish show 
signs of great distress by keeping their mouths out of the water and 
sucking in air. This denotes the exhaustion of the air or free oxygen 
