It is now time to begin to think about what shall be kept in the aquarium. At the bottom a layer of sand, the cleaner the better, two or three inches deep will be needed. A few stones, not too large, may be dropped in on top of this first layer, to make it more natural. The water plants come next and will thrive best if planted securely in the sand. The most difficult thing is to get the water in without stirring things up. A good way is to pour the water in a slow stream against the inside of the aquarium. The best way is to use a rubber tube siphon, but even then the water ought not to flow from a very great height. If the aquarium is large, it had better be put in its permanent place before filling. The aquarium will soon be ready for snails, polliwogs, and what ever else we may wish to put into it. In the course of a few days the plants will be giving up oxygen and asking for carbon dioxid. [Illustration: _Fig. 61. Duck-weed._] _Plants that thrive and are useful in aquaria._--Many of the common marsh or pond plants are suitable. The accompanying illustrations show a few of these. Nothing can be prettier than some of these soft, delicate plants in the water. The eel-grass, or tape grass (Fig. 60), is an interesting study in itself, especially at blossoming time when the spiral stems, bearing flowers, appear. Any who are especially interested in the life-history of this plant may read in reference books a great deal about what other observers have learned from the plant concerning its methods of growth and development. The best that we learn will be what the plant itself tells us day by day. Some of the best reference books on both plant and animal life are found in the New York State Teachers' Library and can be obtained by teachers through the school commissioners. [Illustration: _Fig. 62. Water plants._] Every boy and girl who likes to taste the fresh, peppery plants which they find growing in cold springs, knows watercress. If the aquarium is not too deep, this plant will grow above the surface and furnish a resting place for some snail which, tired perhaps by its constant activity, enjoys a few minutes in the open air. Duck-weed or duck's-meat (Fig. 61) grows on the surface, dangling its long thread-like roots in the water. A little of it is enough. Too much would keep us from looking down upon our little friends in the water. The parrot's feather (Fig. 62, A) is an ornamental water plant that can be obtained from a florist; a plant that looks very like it grows in our ponds. It is called water-milfoil. The water purslane, B, or the common stoneworts, _Nitella_ and _Chara_, D, E, the waterweed, F, and the horn-wort, C, appear graceful and pretty in the water. If you do not find any of these, you are sure to find others growing in the ponds in your neighborhood which will answer the purpose just as well. [Illustration: _Fig. 63. Snail._]