Making a living ecosystem inside a bottle with fish, plants and all other forms of life necessary to maintain a healthy ecosystem is a great way to help form an understanding of the necessities of life. Whether for a school project, or just for fun, building yourself a working example of a contained ecosystem can be done with some commonly found supplies and a trip to your local pet store.
Rinse out a large glass bottle with some hot water to ensure there are no residual chemicals or bacteria in it, then fill with filtered tap water 3/4 full and set aside. The bottle should be large enough to hold at least 1/2 gallon of water.
Rinse some gravel in a strainer under a tap to clean it of any dust or smaller gravel particles that may cloud the water in your ecosystem. Once the gravel is thoroughly clean, pour it into the bottle.
Insert the fresh water plants into the bottle and bury their roots into the gravel.
Put your plecostomus fish and other small fresh water fish into the bottle and then replace the cap over the opening of the bottle. Plecostomus fish, otherwise known as sucker fish, will clean the inside walls of algae, keeping your ecosystem clean and functioning. Keep the bottle in an area that receives some natural light, but not direct sunlight, as you do not want the temperature of the water to be increased.