EECAPITTJLATION. 305 



18. No aquaritim is likely to do weU unless the representa- 

 tives of the vegetable and animal world which it contains 

 are properly balanced : that is to say, that there is a suffi- 

 ciency of oxygen-yielding weed. This balance may be known 

 by the growth of the plants, by the health of the animals, 

 and the brightness of the water. 



19. The water of a properly -balanced aqnarium need never 

 be changed — except, of course, under the special circum- 

 stances which have already been referred to. A little water 

 must be added occasionally to make up for the loss by 

 evaporation. 



20. Aquatic animals shoiild never be crowded together in 

 an aquarium. It is always wiser to have too few animals 

 than too many. When fish are swimming with their heads 

 close to the surface of the water, it may.be taken for granted 

 that there are too many in the tank, or that the temperature 

 of the water is higher than it should be. Under these 

 circumstances, some of the fish should be at once removed, 

 or one of the artificial means of aeration previously described 

 should be resorted to, or the method (which has also been 

 described) of reducing the state of the water to its proper 

 temperature should be employed. 



21. Directly a fish or any other animal is seen suffering 

 from the dreaded fungus, it should be removed from the 

 aquarium. If a cure is wished for, the sufferer should be 

 placed (as already desciibed) in running water, or in water 

 in which chloride of sodium has been dissolved, in the propor- 

 tion of a tablespoonful of the salt to half a gallon of water. 

 A fish should be kept in this solution until it turns upon 

 its back, when it ought to be at once placed in fresh water. 

 This operation should be repeated once or twice every day 

 until a cure is effected. A beetle might be kept in the 

 solution for at least half a minute. 



22. All the animals and plaits which can be kept in an 

 aquarium are interesting and instructive; some, no doubt, 

 more so than others. Nothing, however, should be discarded 

 as useless or uninteresting, for everything has a duty to perform 

 in the world. 



X 



