18 THE FRESH-WATER AQUARIUM. 
stone may be built up into a dark arch, or piled up after 
the fashion of a cromlech—one flat piece resting on two 
or three vertical pieces, so as to form a rude table-like 
structure. ‘These may be fixed firmly in the places they 
are intended to occupy by means of Roman or Portland 
cement, which can be purchased at any building yard. 
The cement should be made into a stiff paste, and worked 
into the form required. Indeed, the rockwork may be 
wholly composed of such cement, especially if it is to have 
the form of an arch. The most important matter in the 
construction of rockwork is to give it a natural, rugged 
appearance, and to avoid loading the tank with superfluous 
weight. JI have seen large shells and branches of coral 
in fresh-water tanks, and always thought the spectacle 
disgraceful to the owner. In a marine tank, such things 
are proper enough. Whatever is done should be made 
secure, the pieces of stone well embedded in cement, and 
the whole firmly united. The tank must be well seasoned, 
be frequently filled and emptied, to dissolve out any free 
salts before being put to use. 
The Bottom must be composed of coarse river sand 
and small pebbles, the whole well washed before being 
introduced to the tank. Mr. Gosse condemns red sand 
and silver sand, as certain to stain the water. But I have 
two tanks now at work, both bottomed with such material, 
and the water preserves a crystalline brightness. I have 
also a marine tank, in which the bed is formed of common 
silver sand and garden pebbles: it has been in use nine 
months, and with no unfavourable results. . In each case 
the sand was washed till the water could be poured away 
