32 PRINCIPLES OF THE AQUARIUM. 



Another evil threatening all aquaria results from 

 not attending to the temperature of the water. You 

 see tanks placed full in the sunlight of the window, 

 where the inhabitants are most exposed to the 

 light and heat. We have noticed how the light 

 thus received encourages the undue growth of vege- 

 tation ; now we have only to remark on the sickly 

 condition of the water when it is so thoroughly and 

 directly heated by the sun. The temperature is raised 

 far beyond what it could be in a pond or a stream. 

 In the former the sides and bottom are always dark 

 and cool, and in the latter the greater ease with which 

 the sun can heat the shallow water is compensated 

 by the constant change of the water in running. In 

 an aquarium placed in the full sunshine it is evident 

 that the animals and plants alike are exposed to most 

 unnatural conditions. With the elevation of the tem- 

 perature there is attended a less capacity for the 

 water to contain the mechanically -mixed oxygen 

 given off by the plants, or even to absorb it from con- 

 tact with the air at the surface. Animal matter de- 

 composes more readily, and thus the water becomes 

 sooner fetid. What the student ought especially to 

 observe, therefore, is that his fresh-water aquarium is 

 placed where the temperature of the water -varies as 

 little as possible. It ought never to fall below 40 

 or rise higher than about 60, if he desires to keep a 

 healthy stock of animals and plants. 



A well-constructed aquarium ought to continue 



