I apologize for not featuring Bottle Biology sooner. This excellent resource has been a staple in my supplemental elementary science materials for many years.
Bottle Biology is “an idea book for exploring the world using soda bottles and other recyclable materials.” The ideas include clear directions for how to convert 2 liter bottles into a variety of aquariums and ecosystems.
Bottle Biology 2nd Edition is available for preview on Google Books. The preview includes most of the “how to” section of the book.
The Bottle Biology Website also has many helpful resources:
- Bottle Basics– description of how to make the different bottle biology systems
- Column Investigations– decomposition columns, TerrAqua columns, and more
Annenberg’s Learner.org has a Bottle Biology page with similar information and some excellent extention activities like the “get a life” necklace.
So ask your students to start saving 2 liter bottles and before you know it your class will be filled with bottle biology.
Science for All 
Also important to note that many of the experiments can be scaled down into smaller containers. Many (elementary) teachers won’t do these experiments simply because of the large amount of flat space needed to house the 2L bottles.