THE FRESH-WATER AQUARIUM. 
37 
aquarium. Either is very apt to spoil the water after remaining 
in contact with it a short time. Coarse sand is, to be sure, some¬ 
times used when we have animals in the tank whose nature it is to 
burrow, but even then only in a small quantity placed near a cor¬ 
ner of the tank. Some of the small lilies grow better if they 
have a cubic inch of peat attached to their roots. This small 
quantity does not injure the water, however long it may remain in 
it, and is often very useful. In general, however, if the plants 
are placed right side up, among small stones about the size of a 
fresh pea, they will grow to any extent, seldom throwing out roots 
of any kind. 
We want, then, a layer of small stones on the bottom, about an 
inch in thickness ; this will be sufficient in which to bury the ends 
of the plants, and to conceal all the sediment which may collect, at 
the same time giving depth enough for the mussels to burrow in. 
The stones used with tar for the tops of houses are about the 
right size for this layer and on the top of it, some larger stones 
about the size of an almond may be scattered here and there. As 
to the color of the stones, this ma}^ add greatly to the effect. If 
we can have the patience to pick out for ourselves the white and 
variegated stones from the beaches, we shall be amply repaid by 
their appearance in water. White stones give a brighter look to 
the inside of the tank than dark-colored ones, and they show off 
the green plants much better ; but they also show the green con- 
fervoid growths growing upon them much sooner than dark stones, 
and are much harder to clean after they once become green. 
This difficulty of cleaning can be remedied by having two sets of 
stones, one being buried in damp sand while the other is in use. 
Were the beautiful stones of almost fabulous brilliancy which 
cover the San Mateo beach, near San Francisco, as common on 
our shores, we should have a famous groundwork for the aquarium. 
After the layer of stones has been evenly spread upon the bottom 
of the tank, we may arrange the rock-work in the centre in the 
following way, which seems to be a good one, because by it we 
avoid using cement, which makes a tank look altogether too arti¬ 
ficial and we get a strong piece of work, giving sufficient shelter to 
the animals and one that will not be likely to fall down and injure 
the glass of the tank. It consists, essentially, of a series of three 
stone bridges, the one above being smaller than the one below. 
If the tank is small one or two bridges may be all sufficient. 
