30 PRINCIPLES OF THE AQUARIUM. 



with animals. Moreover, when the first symptoms 

 set in you can neutralise them by adding another 

 animal a small fish, a young newt, a few tadpoles, 

 or two or three water snails, adding them one by 

 one, and waiting to see the results. In this manner 

 you proceed as a chemist does when weighing some 

 valuable or important medicine. He trickles a little 

 at a time until he has attained the exact weight to a 

 feather. Excessive growth of plants may be kept 

 down in one or two ways. First by modifying the 

 light. Owing to the strong desire to see as much as 

 possible of what is going on in the aquarium, many 

 people expose it as much to the light as they can. 

 And, as they have perhaps fallen into the other 

 mistake of constructing three, if not four, of the sides 

 of glass, it follows that the amount of extra stimulus 

 to which the plants are exposed far exceeds that 

 which influences them in a state of nature. In a pond 

 all the sides are dark the light can only get into the 

 water from above, or through the surface. In a river 

 or stream the sides are always dark, and, though the 

 light can reach the bottom from behind and in front, 

 it passes wholly from overhead. When there is too 

 much glass used in the construction of an aquarium 

 there is a temptation to use the glass : this means 

 exposing the aquarium to light, so that the latter 

 passes completely through it on every side. For this 

 reason bell-glasses are specially to be shunned for 

 fresh-water aquaria. The young beginner has only to 



