While largely harmless, algae can be an unsightly nuisance. Algal spores live everywhere, blown by wind in their dormant states. However, these spores can rapidly grow into thick algal growth under the right circumstances. There are several methods of controlling algae in small containers like five-gallon water bottles. However, each method has pros and cons of its own.
Chlorine bleach will rapidly kill and dissolve algae. However, you need to be careful when using this method, since bleach is corrosive and toxic. Even diluted, bleach can discolor plastic and corrode metal. Additionally, if you plan on using your bucket for animal care, like aquarium water changes, you cannot use bleach. If these are not concerns for you, just carefully mix two tablespoons of bleach per gallon of water into your bottle.
Many stores sell various chemical products to help control algae, either for aquariums or for outdoor water features.A wide variety of chemicals are used in these products, but most are less corrosive than chlorine bleach. Always follow the manufacturers' instruction and cautions for these products. Some can be toxic to plants or even animals, and may not be appropriate for all applications.
Under some circumstances, you can use living things to help control algae. With aquariums, algae-eating fish may be used. However, using animals can be a dicey proposition, since animals need more food than algae to survive. Additionally, animals produce biological wastes of their own, which can make algae worse, especially in the confines of a five-gallon bottle. Invertebrates like shrimp or snails may work better. However, plants are one of the better organisms to control algae, since they thrive under similar conditions and absorb nutrients faster than algae can, effectively out-competing them.
Deprivation is a simple method to control algae. Algae needs three things to thrive: water, light and nutrients. If you do not add any kind of nitrogen or phosphorus to your bottle, algae cannot survive. However, the tap water in some areas already contains these nutrients. But if you block out light, algae cannot survive. Blocking out light, or using dark-colored bottles with lids can reduce the amount of light that reaches the water, light-starving the algae.