If you have water bottles that have water in them all the time, you may start to get algae growing in them. This happens to me with the water bottle that I keep in my hamster cage. If you want to clean the algae out of the water bottle, pour a little bit of water in the bottle and add a couple teaspoons of rice. Then close up the bottle and shake it vigorously for several minutes. Once done, pour out the water and the rice and rinse the bottle several times. You'll have a clean, algae-free bottle. This works cause the rice acts like an abrasive when it is shaken to scrap off the algae (and other things growing in the bottle) from the sides of the bottle.