Lots of factors can turn pond water green or brown, so you’ll need to do some searching to discover the exact cause. Brown farm pond water can be caused by soil erosion muddying the water, tannins that come from plant matter that is breaking down, nuisance fish like goldfish, carp and bullhead that can muddy the waters, and sludge that comes from decaying organic matter on the bottom of the pond. Each of these things will require a different fix—for instance, removing nuisance fish if they are muddying the waters.
Green pond water comes from too much algae, which is a signal that the pond has a nutrient imbalance that is causing the algae to thrive. You’ll need to identify the cause of the nutrient imbalance, which could be too many fish adding their waste to the water, too much sludge accumulation, or too few plants to use the nutrients in the pond.
