How do I stop my pond going green?

"My pond has gone green again! This happens every year and nothing I do seems to fix it. What’s going on?"

This is without doubt the most common problem of any pond, be it natural or man-made. There are a lot of factors to consider when trying to solve a persistent green-water problem so we shall address a few possibilities here. If you have already tried one particular option, move on to the next one and so on in turn until the problem is solved.

So why does green water occur in the first place? Well, the green murk is actually photosynthetic microscopic algae that are using the light from the sun and nutrients present in the water to grow and reproduce. In a natural system, the spring blooms of these algae are very important for consuming the waste products that have built up over winter. Once the algae has bloomed, small crustaceans such as Daphnia (water fleas) quickly start to feed on the algae and in the process, the pond becomes crystal-clear. The algae and its planktonic consumers then live in a balanced ecosystem, controlling each other’s populations. It is at this stage that the pond can quickly plunge back into a green soup should anything disrupt the balance. Obviously, man-made ponds are not so simple, but a balance must still be found.

Firstly, there are many products you can buy that are designed to solve green-water problems. Speak to your local Maidenhead Aquatics store about which product they recommend. Any treatment used in a pond (or aquarium, for that matter) must be dosed according to the manufacturer's instructions. Buying a cheap bottle of treatment that won’t actually last long enough to complete the full dosage for your pond is a waste of time and money. Sometimes you will find that a pond will benefit from a single treatment at the start of the season and then not need anything else adding to it after that.

Next, we must think about where the algae are getting the necessary materials for growth, and this means light and nutrients.

With regards to light, it can be challenging to change what your pond receives. Also, it is important to accept that green-water issues in spring and sometimes late autumn is completely natural and not something to worry about. As moving the pond is not really an option, adding shade by planting lilies and marginal plants can be effective. If you are thinking about building a new pond, make sure you consider just how much sun it is going to get. A pergola, fence or shed can sometimes make the perfect shadow onto a pond, particularly if it's destined to house large carp which will often make planting awkward.

The major control we have on our ponds is how much waste (think of it as fertiliser) goes into them, and more often than not, this is the limiting factor for green-water outbreaks.

Good, adequate filtration is a must. If you have fish and want clear, healthy water, then realistically you need a filter. Fish create huge amounts of waste from the food that we give them in addition to what they scavenge from the pond themselves. Don’t forget that Goldfish will feed on beneficial planktonic bugs, so even the introduction of a single fish into a well-established pond can upset the balance and lead to a green-water issue. If you are not sure what filter you have or how to correctly size one up for your pond, please speak with a member of Maidenhead Aquatics staff. For 8 out of 10 pond owners, installing a decent filter system will cure any long-term green-water issues and maintain a clear pond for years to come.

To ensure that your filter removes the algae, the addition of an ultraviolet clarifier is needed and in fact, many filter systems come with UVs built-in. The radiation from the UV lamp destroys the genetic material of any cell that passes through it, thus killing the microscopic algae and helping to win the war with green-water issues. Do remember that a UV alone cannot solve the problem for you, however - it must be run alongside adequate mechanical filtration and must have a new lamp fitted once every year. Just because the bulb continues to light up, it doesn’t mean that it is still effective. Unlike visible blue light, the UV radiation that is emitted will decay over time until it is essentially non-lethal to algae. As UV light is damaging, follow the manufacturer's safety recommendations and never look at a burning lamp. Translucent plumbing or similar features normally allow you to check the lamp is functioning.

If your pond is not in a position to fit large filters, then you can use nature's own. One of the best ways to combat algae is to have a well-planted pond. Plenty of oxygenators and fast-growing rushes win the battle for light and nutrients, keeping the pond clear even during the sunniest of months. However, if you are looking to employ this method, you must properly invest in your pond plants, as a single sprig of pondweed won’t cut it. Remember too that larger fish like koi will simply ravage anything they can get their barbels on, so this method is really best suited for natural wildlife ponds or ponds with low stocks of fish

Finally, you are left with a harder decision. If you have an adequate filter for the size of your pond but continue to suffer from green-water no matter what you do, you may have to look into the stocking and feeding of the pond. If you are serious about sorting out your green-water problem in the long term, enlarging your pond or reducing fish numbers can make a big difference. The same is true for feeding - ensure that you only feed what your fish can eat in ten minutes. After that, use a net to scoop out any uneaten foods.

If you cover and address the issues above then you will practically guarantee yourself a clear and healthy pond. If you have any further issues, please speak to a member of Maidenhead Aquatics staff at your local store - we spend a lot of our summers talking about algae problems!