Building a pond ecosystem in an empty 2- or 3-liter soda bottle is a simple project for elementary kids of all ages. The easiest way to acquire materials would be from an aquarium store, but allowing kids to gather their own materials is also possible if there's a pond near your school. If purchasing materials, ask the aquarium store for bunches of water plants called elodea or anacharis, small snails and feeder guppies. It's also recommended that the water used comes from the aquarium to ensure that the proper microorganisms are present in your ecosystem. If collecting materials locally, remind kids that the pond should not be disturbed too much and that they should only take what they need. Help your students gather pond water, dead leaves or other plant matter, snails, plants and fish or small crayfish if possible. You will also need small and medium-sized rocks for your ecosystem, either purchased or collected and cleaned.
Once materials are gathered, fill the bottle three-quarters of the way to the top with water. If using tap water, be sure to let it sit for at least 24 hours to dechlorinate. Start by adding 1 or 2 inches of rocks to the water, placing smaller rocks and gravel in first, then adding medium-sized rocks. Next, add a small amount of dead plant matter along with the elodea, making sure the strands are separated. Once these parts are in the bottle, add the snails and lastly add one fish to each bottle. The ecosystem needs to settle with the top of the bottle off for 24 hours.
Since light is an essential part of an ecosystem, where you place your ecosystems in the classroom matters. Ecosystems should be placed in an area that gets a few hours of direct sunlight each day. They should be placed at or near student eye level to make observation easy, but they need to be somewhere where they won’t be easily knocked over or disturbed as well.
As the ecosystem sits, highlight things that students should try to observe. Students should see oxygen bubbles and new growth on the water plants as well as algae growing. They can also look for snail eggs and other microorganisms, and watch the fish as it searches for food. Allowing students to observe the ecosystem through a magnifying glass or microscope will enable them to make closer observations.