One of the most common annoyances for the water gardener or pond keeper is green water. This problem is particularly prevalent in spring or early summer.

The source of the green colour is a microscopic algae which is so small that it cannot be removed through normal filtration. This algae thrives in spring and summer as there are increased nitrate levels in the pond as a result of increasing fish activity and also the increased sunlight which encourages plant growth. This is also a particular problem in new ponds where the chemical balance of the water has not yet been established.

In terms of preventing green water there are two stimuli to manage, these are nitrates and sunlight. Limiting nitrates in the water can be done by limiting fish stock, removing decaying matter such as leaves and excess fish food and also ensuring that the pond is well planted. Plants will absorb nitrates as part of the natural cycle and it is the excess nitrate that algae so readily exploit. Managing sunlight must be approached in a realistic manner. Providing shade over and around the pond will have a beneficial effect on the pond habitat. Ensuring that pond plants themselves cover a substantial proportion of the water’s surface will also restrict the available sunlight for algal growth.

These preventative steps will certainly limit the possibility of green water but will not guarantee it never occurring. These days, there is an easy solution to the problem with the use of UV clarifiers. UV clarifiers work by killing the algae and causing the dead algae to form clumps which can then be removed through filtration.