PREPARING THE TANK. 31 



is sometimes recommended, and I have used it for this 

 purpose : but it requires a great deal of washing to make 

 it fit to be placed in an Aquarium, • as, after even a 

 dozen good washings, I have found that 'it still slightly 

 tainted the water, and even if wc could perceive no color 

 imparted, yet there is apt to be lime, or some marine 

 salts, dissolved out after a time, which renders the water 

 injurious to the fish and other inhabitants. I, therefore, 

 cannot very highly recommend silver-sand for a bottom to 

 the tank, though it may answer when so used. And as a 

 general. rule, all colored sands should be rejected as worse 

 than useless. The common red gravel, which would look 

 well in a tank, must not be used, for the reason that it 

 is ferruginous — that is to say, it contains iron, which 

 would soil the water of a red color. When saying that 

 silver-sand was not applicable to Aquarium bottoms, I 

 meant more particularly fresh-water tanks : for I have 

 used it in marine ones, after well washing it, so as to 

 remove all dirt, and with advantage, as there arc several 

 crabs and other animals that delight in burrowing in it ; 

 still, even after well washing, this sand is apt to retain 

 some of the salts it takes up on the sea-shore, and which, 

 in a marine Aquarium, would do no harm, yet would, in 

 a fresh-water one, render the water brackish, and so de- 

 stroy the plants certainly, and perhaps several of the 

 animals. We therefore procure a perfectly clean specimen 

 of sand, place it on the base of our tank, to the depth of 

 at least half an inch, though I have found that,, generally, 

 an inch is the best depth, and leaving it in some places 

 exposed, place over the same a layer of small pebbles, to 



