A livestock water tank that has turned green with algae and other aquatic plants is both unattractive and unhealthy for the animals using it. Fortunately there are substances, including bleach, that can be used to treat the water. The trick is to know how much to use to keep the water tank free of the algae.
Chemicals to Use

Copper sulfate, sometimes called bluestone or blue vitrol, is sometimes used to keep water tanks clear. Caution is urged if sheep are being watered from the tank as they do not tolerate copper which accumulates in their system. Rate of application is one eighth of a teaspoon per 100 gallons of water.
Chlorine bleach, 5.25 percent sodium hypochlorite, is more commonly used to keep stock water tanks clear. While this is common laundry bleach the unscented variety is recommended by the University of Missouri Extension Office. Rate of application is 2 to 3 ounces of bleach per 100 gallons of tank capacity each week.
Calculating Tank Size

If the capacity of the stock tank is unknown a simple mathematical formula can be used to estimate the size of the tank.
Measure the tank in feet.
For rectangular or square tank the formula is length of the tank times the width of the tank times the depth of the tank times 7.5. A tank that is 6 feet long by 2 feet deep and 2 feet wide would have an estimated capacity of 184 gallons (6x2x2x7.5=184).
For round tanks the formula is the diameter times the diameter times the depth times 6. A round tank that is 5 feet across and 2 feet deep would have an estimated capacity of 300 gallons (5x5x2x6=300).
Other Tank Cleaning Ideas

Algae grows more rapidly in warm water. Shading the tank will keep the water cool and slow the growth of algae.
Add goldfish to the tank to eat the algae. The University of Missouri suggest four to six goldfish per 100 gallons of water tank capacity but cautions while you are removing algae from the tank you are adding fish feces.