Algae in stock and nurse tanks can be a nuisance for both animals and for chemical application especially with hard water.  Adding copper sulfate to the water will take care of the problem until the tank is refilled with fresh water.  The process must then be repeated.

Dissolve 1 ounce of copper sulfate in 1 pint of water in a glass jar. Add 0.25 pt/1000 gallons of water.  The copper sulfate will not affect herbicide and weed control.  An alternative for nurse tanks is to paint the entire tank black.  This eliminates the sunlight requirement which prevents algae growth.

From May 25, 2006 North Dakota State University Crop and Pest Report