48 FISH CULTURAL ASSOCIATION. 
troduced, are beneficial in keeping this growth down to some 
extent, but are as likely to eat the plants which they should not 
as all other things are to work against our wishes. 
The well-known Venus, ear, Halotis tuberculata, is a favorite 
in English tanks, and eats conferva clean, and in the two best 
aquaria in that country the tanks are never cleaned on the in- 
side, yet they look as if that operation had just been per- 
formed. 
In a French book on acclimatization, by H. de la Blanchere, 
it is stated that the more the plants belong to the inferior or- 
ganisms the greater their oxygenating power, and that crypto- 
gamic plants have a greater power of vivifying water on account 
of their greater evolution of oxygen, and also that M. St. 
Hilaire has found that conferva in the aquarium of the*®Société 
d’Acclimatation in Paris produces a constant and enormous evo- 
lution of oxgen. 
Such is the influence of light upon vegetation that in my 
own fresh-water parlor-tanks I have found it necessary to shield 
them at night during the periods of full-moon in order to check 
the growth of conferva, and ina public aquarium it is hardly pos- 
sible to have it too dim for the well-being of the inhabitants, 
few of whom live in strong lights. No more light should be al- 
lowed than that which comes through the water, as then the 
visitor, standing in obscurity, can readily see what would other- 
wise be indistinct ; hence all attempt at ornament or display 
outside the tanks is useless. | 
These remarks, so far, have been intended more for those 
unfamiliar with the subject, and really contain little that is new 
to the few who have studied it closely ; but in order to render 
what [ intend to say intelligible to the former class, I wish to add 
the well-known fact that in aquarium-keeping, the longer the 
water is used the better it is, and that the introduction of new, 
or fresh water, is often fatal. To sum it up in the fewest 
words, there is not a drop of new water in the world; it has 
been breathed over and drank over millions of times; the 
sun draws it up toa certain height only, and it is blown over the 
land and precipitated in rain, and then returns to the sea. An 
aquarium, such as I am describing, isa miniature world; the reser- 
