Green water is usually caused by algae.
The clarity of your pond water can be affected by several factors. Firstly, if the water is green then immediately the first thought should be is my UVC clarifier working? Perhaps the lamp needs changing which is recommended to be done annually, or another consideration is if your UVC clarifier is big enough for your pond and if so an upgrade or additional UVC clarifier may be the answer. If you do not have a UVC clarifier then getting one installed is the best solution to this problem. There are also several “green water” treatments available on the market in shops however long term a UVC clarifier is truly the best solution. Another thought is whether the water parameters are at healthy levels. If left unchecked nitrates and phosphates in the water will steadily increase which in themselves directly feed the like of algae so a simple series of water changes to bring these back down to acceptable levels can help bring the water clarity back.
Now if your pond is murky with silt suspended in the water column then the initial first thought would be the base of the pond has buildup of silt, sludge and debris so therefore does the pond need vacuuming or even does it need a full clean out. The other thing to consider is your filtration system. Does it need some attention in the form of cleaning out, changing the sponges etc. Or is it not large enough for the pond, perhaps the baby Koi you purchased last summer have now grown so much that the volume of waste they produce now compared to their smaller, younger selves are simply too much of a strain on the original filter system. Once again there are “sludge buster” and “silt remover” chemicals readily available on the market in shops but this would really be a plaster on a bullet wound and instead dealing with the route problem will bring the best and longer lasting results.
