This is the time of year, the calls come in asking about moss and algae growth in ponds and stock tanks.  While there are many ways to control, it is constant.
 
One of the easiest and least costly is to purchase some goldfish, and let them control the green stuff.  Many times the tank is not deep enough for overwintering, so you can either bring them inside or not.  My suggestion is to put a milk crate or larger chunk of pvc pipe in the bottom of the tank, so the fish can hide from would be predators.
 
You can also use a bleach solution.  Using 1 ounce of household bleach per 1550 gallons of water should clear it up, without harming livestock.  The Dairy Herd magazine quoted, David Beede, Dairy Nutritionist at Michigan State University, “every week add two to three ounces of household bleach to every 50 gallons of water.  That would be a bit stronger!
 
Copper sulfate is a good ole standby.  Dissolve 1 ounce of copper sulfate in 1 pint of water.  Then add 7.5 tablespoons of this mix to each 1000 gallons of water.
 
There is a commercial product called Stocktrine.  You will probably have to special order this in, follow label directions.
 
To ensure cleaner tanks longer, clean them before treatment!