CHAPTER yi. 



FRESH-WATER FISH AND MOLLUSCA. 

 THEIR DESCRIPTION AND PECULIARITIES. 



The tank is now presumed to be fully prepared for the 

 introduction of animal life. In order to keep it in that con- 

 dition, it becomes obligatory upon its proprietor to supply 

 it with a few common pond-snails. These will consume 

 the decaying vegetation. They will even do more than 

 this, as we shall see presently. If your tank be placed in 

 too strong a light — and particularly if placed in such a 

 position as to catch the direct rays of the sun — you will 

 soon observe its effects both upon the plants and the wa- 

 ter. A greenish, slimy kind of mucus will soon be found 

 adhering to the sides of the glass, and obstructing the 

 view, besides barring the passage of the light. This will 

 become gradually denser and denser until thoroughly in- 

 tolerable. This deposit is, in fact, a new vegetable 

 growth of an infinitely small species of alg(Bj or vegetable 

 weed, and is termed the confervcB. With too little light, 

 your large plants languish ; with too much, your Aqua- 

 rium fills with conferva. Hence the necessity of observing 

 a prudent medium. With the growth of the confervcBj 



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