Why is the water in your pond or pond green?

Why and how to remedy green water in your pond?  This green water is mainly due to the appearance of microalgae. In fact, it is phytoplankton: single-celled algae that should not be confused with filamentous algae.

Green water is very often present in new ponds, and more particularly in those that have nutrient-rich water. 

Why is the pond water green?

Green water mainly appears during the spring. As temperatures start to rise around 14 degrees, phytoplankton forms. Heat therefore amplifies the imbalance of the pond ecosystem, but it is entirely natural.

In addition to strong sunlight and warming water, other causes can come into play, such as a nitrate spike, often due to:

• Overcrowding of fish in the pond, a lack of plants,
• An accumulation of organic waste often due to leaves falling in autumn and winter.
• Also a lack of oxygen in the pond and stagnant water.
• Another cause is the use of a filter that is not efficient enough for the size of your pond and insufficient regular water changes. You need to change 10% of your pond's volume per week. Green murky water is not toxic in itself to your fish, but it can cause strong variations in oxygen and pH at night. It also helps protect them from sunburn.

Proposed solutions to prevent green water in the pond

The use of products

The first solution and often the first reflex is the use of a product. If you look closely on the Internet, the advice you find is often written by manufacturers, who are looking to sell their products.

That said, before any intervention, the ideal is to work on the causes that led to the appearance of green water. This is an important step because, obviously, the phenomenon will probably return as long as the real causes have not been found and treated.

The use of pumps and filters

There are other systems that allow you to mitigate this effect: the use of a pump and a filter, increasing the number of plants (also annoying) and trying to create as many waves as possible on the pond.

⭐ This may interest you: we show you how to choose your pond pump.

In summer, lakes and ponds are green because, obviously, there is no filtration system. A fish population that is difficult to regulate, very often lacking oxygenating plants and very insufficient water circulation. Conversely, rivers and waterfalls are not conducive to opening, because the water is not stagnant and is not in a closed circuit.

⭐ This may interest you: our detailed article explains how to choose a filter for the garden pond.

The use of a UV sterilizer

So the use of a UV sterilizer is another solution that is the most effective of all. However, the downside is the electricity consumption, indeed, and you have to count about 3 to 5 Watts per cubic meter of water, depending on the dimensions of your pond.

⭐ This may also interest you: our collection of ultraviolet devices

To date, this is really the only solution we can call truly effective to fight green water.

Warning: it requires the use of a pump, of course, adapted to the flow rate of your UV sterilizer. In parallel, it is the best ecological solution compared to the use of algae treatments.

Finally, the electricity consumption of UV will cost you much less per year. Unlike the use of multiple products that will only play temporarily, knowing that these products will also have an impact on the fauna and flora.

In what order to test the water of a green pond?

Before choosing a solution, a few simple measures allow you to know what feeds the algae. Proceed in this order:

• Hardness (GH and KH): water that is too soft, with a low KH, makes the pH unstable and promotes imbalances.
• pH: check it next, preferably always at the same time, because it varies between day and night in green water.
• Nitrates and phosphates: if the water remains green despite correct hardness and pH, it is often them that maintain the algae.

Light also plays a role: a pond in full sun all day, without plants to shade it, turns green faster. Floating or bank plants that cover part of the surface limit this effect. To go further on phosphates, read our article Phosphates in the pond: why phosphorus feeds the algae, and how to lower it sustainably.

Conclusion

Green water is not toxic in itself for your fish or your pond, but it can cause strong variations in oxygen and pH at night; for the rest, it is mostly the aesthetic side of the thing. It also allows to hide your fish from the view of predators.

⭐ This may interest you: the list of oxygenating plants to keep a balanced pond

If you are more into organic or nature, you will leave your pond with green water. However, if you really want to have crystal clear water, the ideal would be to go through the UV sterilizer which is one of the best alternatives.