Your water can turn green for a variety of reasons; all need to be considered in order to transform your fish pond into one with crystal clear water that can be enjoyed by all.
Green water is not usually harmful to fish but it does not give the attractive appearance you were intending, also it is nice to be able to clearly see the fish in your pond.
The water will turn green because of excess growth of algae the main factors for this growth are; The pond is new - When you build a new pond it will take some considerable time for the water to settle and the correct amount of microorganisms to grow in order for the water to become balanced and stable.
Your pond is overstocked - If you have a large number of fish you may find that the waste they produce is too much for the pond to cope with providing nitrate waste that algae will thrive on.
If you do have a lot of fish you will need to ensure that you have installed a good filtration system to remove the excess waste.
You are overfeeding your fish- Be careful not to overfeed your fish as this will cause the water to become unbalanced.
Feeding levels differ for each season so check to make sure you are feeding the correct amounts Too much sunlight - sunlight triggers rapid growth of algae.
Ponds that have natural shade seem to develop less algae.
You can provide shade by wise planting of surface cover plants such as water lilies and pond hyacinths.
Large plant surrounding you pond are also good.
You will just need to be conscious of leaf loss in the autumn as this can have a detrimental effect on your pond.
You can also install a UV Filter that will kill algae or use chemicals that you can purchase from your local garden centre whilst these provide a solution there is no better answer than time as you new pond will take time to mature and become balanced.