Rock-work is not only ornamental, but useful, as it furnishes hiding-places for the animals - all of which love seclusion at certain times. In selecting rocks, see that all those containing lime and other soluble matters are avoided. To determine this, pound some of the rock up and mix it with clear rain-water in a tumbler. After standing a day or two, evaporate a little of the water in a watch-glass, or even on a piece of thin window-glass, and if it leaves a considerable deposit the rock is soluble and consequently bad.
By keeping a sharp lookout we may often find rocks of a very picturesque form, and having little holes or pockets in their sides. These holes may be enlarged and used for holding small plants, which will grow in them freely.
Avoid all artificial nonsense like earthenware castles, submarine hermits, glass swans, cast-iron frogs, and the like. No person of taste would allow any such childish make-believes to have a place in an aquarium. So, too, avoid the incongruity of placing sea-shells and coral, however pretty and handsome, in a fresh-water aquarium. A sea-shell in a marine aquarium may be not only appropriate, but useful, as many animals use these deserted shells for burrows; but in a fresh-water aquarium they are entirely out of place. And yet we have seen not only large sea-shells in a fresh-water tank, but of a whole herd of china (not Chinese) cattle placed as if grazing on the bottom!