Getting a pond balanced is an interesting ecological problem. The fish are fed and excrete into the water making it a dilute fertiliser which encourages plant growth. If there are no higher plants then algae will grow. You have to remove a crop of green plants to keep the thing balanced - the simplest way is to float duckweed or Azolla and skim it off regularly - this removes the nutrients from the water. More attractive is the growth of pond weeds such as Elodea which can be clipped from time to time.
Green water caused by algae can be treated with Daphnia (water fleas) which the fish will eat and they are better than fish food. Snails will cruise the sides and bottom and eat the algae there. You can get all these things from the place that sells ponds and fish.
Hope this helps, Alan