234 PRESH-WATEB AQUARIA. 



Excluding the inhabitants of the sea, all the many and varied kinds 

 of aquatic life may be roughly divided into those which live in ponds, 

 or comparatively slow running streams, and those which live in rapid 

 ones. This division will serve to indicate the reason why some animals 

 cannot be kept in an aquarium, for, generally speaking, it is only those 

 included in the latter group which do not thrive in an ordinary well- 

 managed tank. 



I have been many times asked the question: How often do yon 

 change the water? — and usually meet with expressions of astonish 

 ment when I answer that it is unnecessary to do so at all. Yet, if the 

 inhabitants of the tank are in health this is quite correct, though it is 

 well, as we shall presently see, to have an occasional " clean out." 



There should be a proper proportion of auimal and vegetable 

 life in an aquarium, or rather a preponderance of vegetable 

 life ; for, although such animals as are carnivorous, and at 

 the same time air-breathing, do not directly require it, yet, as they 

 feed on those who are or have been either vegetarians or water- 

 breathers, or both, indirectly they do, as besides forming food-stuff, 

 plants are necessary to render the water capable of oxidising the blood 

 of such of its inhabitants as do not derive their supply of oxygen 

 directly from the air. 



Water absorbs oxygen from the atmosphere by the simple contact 

 of their surfaces, and if the superficial extent brought into contact is 

 very greatly increased, as by the formation of waves, the flowing of a 

 stream, and more especially by violent agitation, such as is caused by 

 the beating of waves on the shore, enough oxygen will be absorbed to 

 supply the animal inhabitants with all they require. As this cannot 

 take place in pools or other small pieces of water, the supply must, 

 consequently, be obtained in some other way. The action of the 

 chlorophyll or the green colouring matter of plants on the carbonic 

 acid gas contained in the water is the most constant method. It 

 decomposes it into its two elements, carbon and oxygen ; appropriates 

 the former to its own use, and liberates the latter for that of animals. 



Hence it will be seen that, although some animals may be kept in 

 an aquarium without plants living with them, it can only be by the 

 aid of extraneous assistance ; food must be placed within their 

 reach ; and the water must be aerated by agitation or some other 

 mechanical means. 



The first requirement is a vessel to contain the water. This may be 

 made of almost any material, but the sides should either be vertical 

 or inclined from the bottom outwards. " Fish globes,' - with the water- 

 line above the greatest horizontal diameter are to be shunned as the 

 very worst form. A slate tank with plate glass front is by some con- 

 sidered the best vessel for an aquarium. I am, however, inclined to 

 think that the use of rough plate glass for the sides and back would 

 he an advantage, but an ordinary Ljlass pan of about sixteen inches 

 diameter and six inches deep will do exceedingly well; such vessels 

 are sold under the name of "pastry pans." I believe, and cost about 



