How do I prevent green water?

Green water can become a nuisance for any water feature or pond; however, this can be simply resolved. It is important to understand what causes the green discolouration to the water and how your pond can be managed to ensure better clarity.

What is algae?

Algae is a term used to cover a range of simple, non-flowering and commonly aquatic plants, with many existing as single-celled forms. It can exhibit itself in several colours however commonly it appears green, which gives the water a pea-soup colouration. Algae will always exist within any body of water, although at small quantities these microscopic organisms may not be apparent, although at larger and denser quantities the clarity of the water can begin to suffer.

What encourages algae growth?

Algae will feed on excessive nutrients within the water, mostly nitrate and phosphate, although sunlight is also required for the organism to replicate. Since sunlight is a factor which cannot be controlled, nutrient content within the pond should be maintained at suitable levels. In order to do so it is important to understand how these nutrients enter ponds and what actions can be taken to control them. Having suitable filtration, both mechanical and importantly biological will also help to keep nutrient levels manageable. Mechanical filtration will remove large volumes of waste whilst the biological filtration allows for completion of the nitrite cycle, ultimately assisting in removing some excessive nutrients from the water. Both nitrate and phosphate are measurable by a standard pond test kit set and controlling them always reduces algae growth.

Nitrate is the last stage of the nitrogen cycle, the biological process that happens in any aquatic system (whether it be a pond or aquarium). Nitrate is considered relatively safe to your pond in lower doses, and in a perfect world, we would carry out small water changes every month with nitrate free water, and keep the nitrate level diluted and under control.

Phosphate is the other nutrient which is within our control. It comes into our ponds from four main sources, all of which we can control to some extent ; soil in planting baskets, fish food, tap water and run off.

Planting Basket Soil in your pond is a first source of phosphate, as it contains low amounts of phosphate. The actual quantity can be quite variable, and garden compost (sometimes recommended in garden books) can have quite high quantities within it. This phosphate will leach out of the soil over time straight into the pond water. ACTION – Using a good quality aquatic soil, (or just gravel if the plants you are growing are low in requirements for nutrient) helps to reducing the availability of phosphate from this source.

Fish food. Phosphorous is put into fish food as part of the diet, being an essential element for fish health. When you feed your fish, some of the food will inevitably not get eaten and processed by the fish, and therefore it again leaches into the pond water. ACTION – use a good quality fish food, and do not overfeed. Feed the fish in small quantities, and make sure all the food is eaten within 3 minutes and not left floating around the pond for a long time afterwards.

Tap Water is also a major cause of phosphate. It is an additive in fertilisers for crop production and gets into our water supply, meaning water changes also add phosphate! ACTION – This one is a bit trickier to deal with, but with a small pond you can add rainwater to your pond rather than tap water. Alternatively, add a phosphate absorbing filter media cannister to your hosepipe when topping up your pond, which will dramatically lower the phosphate added to your pond.

Run-off water can also be a cause of phosphate levels accumulating. Run-off water first washes over your garden, and as it travels the phosphate leaches into the water from the surrounding soil (especially if you add fertilisers to your garden or lawn). Note, this is only a cause where run off water can enter the pond – a raised pond, or one with a solid barrier around the edge will not suffer from this. ACTION – ensure any water from your garden is directed away from entering the pond.

What else can be done to manage nutrient levels?

As part of a healthy pond, it is always advisable to have a reasonable amount of plants within the pond. All the different types of plants – marginals, oxygenators, deep-water marginals and water lilies will all also compete for the nutrients in the pond, and will often be successful in using the nutrients over the algae. Therefore, if you have a reasonable quantity of plants growing in your pond, this will help to keep everything balanced and algae under control.

Good maintenance is also helpful in controlling algae. The less waste and silt there is at the bottom of the pond, the less there is to break down through the nitrogen cycle and end up as nitrate. We recommend maintaining your pond at least 10 times a year, ensuring the filters are kept clean, the pond vacuumed, and water changed (obviously refilling with as low nitrate water as possible). Also ensure the quantity of fish does not get too many for your pond! Goldfish can breed very well in garden ponds, and as a result if they are very successful you end up with too many fish for the pond, meaning you will add a lot more food resulting in higher levels of phosphate leaching into the water.

It is also becoming more commonplace to use phosphate absorbing media within a filtration system, and then replacing this regularly as it becomes full. Although this is not a natural process, it can be quite effective as long as the pond is not too large, otherwise it becomes costly!

Is it time to use a UV filter?

When a pond has been correctly managed and maintained, however algae growth is still problematic leaving the pond with a green colouration, another management technique is to install a UV clarifier or steriliser into the filtration system. This is a sure-fire way to keep pond water clear of algae. A suitably sized UV unit will eliminate any issues with green water.

These UV systems work by having water pass through the unit, which houses an ultraviolet bulb. This bulb produces a high intensity light which kills the microscopic bacteria as it passes through. When deciding on which Unit is the best for your pond the determining factor is the size of your pond. UV bulbs having varying Wattages which affect the power and efficiency of the system meaning that a larger pond will require a higher Watt bulb. UV units are sold individually however most pressurized canister pond filters will have a UV included. If this is the filter used on your pond it is important to know what wattage bulb the unit is intended for and also to insure the bulb is changed approximately once per year. If the filtration system used on your pond is something such as a chamber filter, drum filter, bead filter or one such as a nexus then a UV will have to be installed in-line. Preferably the UV will be installed after the filtration, which allows for the water to have the heavy particulates removed ensuring full efficiency from the UV. This will not be possible for all types of filter though, certain chamber filters will be pump fed and then gravity return to the pond, meaning the UV should be installed between the pump and the filter.

If the above advice is followed and suitable UV unit has been selected, and correctly installed and managed then you will never have to worry or stress about green water anymore! If you would like more detail on differing systems or installation guide for UV units then check out our other blog post, or get in contact with out team and we can advise on what is best for you.

Contact us

The Pond Guys are a pond installation and maintenance business based in Surrey, with teams covering London and the South East.

We are specialists in all types of ponds from ‘state of the art’ Koi ponds through to tranquil water gardens and water features.

Do get in contact for any help and advice, or indeed if you need help with any aspect of your pond or feature.

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