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Use a 2 quart (2 liter) plastic bottle and fill it ¾ of the way with water. You can either use water from a local pond or stream, or from the tap. Pond or stream water is preferable because you will also get small microorganisms that may be present in the water. If you have to use tap water, make sure that you let it sit in an open container for at least 24 hours before using in your ecosystem. The chlorine present in tap water may kill any animal or plant species that you add to your ecosystem. Letting the water sit for 24 hours gives the chlorine time to dissipate from the water. Next, add 1-2 inches (2.5-5 cm) of tiny rocks or pebbles. You should always wash any rocks prior to adding them to your ecosystem. This will help remove any contaminants. At this stage you could also add one dead leaf. This will provide a food source for any microorganisms in the water. You can buy aquatic plants from your local pet supply store. When you add the plants to your ecosystem, make sure to separate them and add them individually. Alternatively, you could pick water plants from a local pond. You can also purchase small freshwater snails at your local pet supply store. Alternatively, you may be able to find water snails from a local pond. Make sure the snails are small enough to fit through the opening of the bottle. Once you have placed everything into your ecosystem, you should wait approximately 24 hours before sealing it closed. This will allow your ecosystem to settle. After 24 hours you can screw the cap onto the top of the bottle. Situate your aquatic ecosystem in a sunny spot. The bottle should receive indirect sunlight throughout the day.

Summary:
Fill the bottle ¾ of the way with water. Add pebbles. Insert aquatic plants. Place snails in the bottle. Wait 24 hours before putting the lid onto the ecosystem. Place the ecosystem in a sunny spot.