Bradshaws Banner

Order Bradshaws Catalog button


Algae & ponds

Ponds & filters

Pond plants

Ponds & water gardens

Pond & fountain pumps

Top ponding tips


Pond liner sizes

Tensor pondvac

Oase Pondovac 3

Hozelock Cascade pumps

Oase pressurised filters

Maximus fountain pumps

Olympus Filter Pumps


Algae In Garden Fish Ponds & How To Control It 100% Effectively In Your Koi Pond

Algae are tiny plants. They like sunlight and grow rapidly when fed nutrients in the form of nitrogen chemicals (like nitrates that develop naturally in all garden fish ponds). Algae therefore need food, oxygen and sunlight to grow and survive.

Like most of us the more food, the more oxygen and the more sunlight the better and bigger the algae grow. In the right conditions the algae grows at an explosive rate - 30 times per hour!! This is why a swimming pool can be perfect one day and the next day it is totally green. You say how to yourself how can this be this possible? - Now you know why.

Blanketweed and String Algae

Not all algae are green and not all algae are minute in size and nor do they all remain suspended in water giving that pea-soup appearance. Some algae join together as a long straight line and create long slimy strands yet the water remains perfectly clear (blanketweed). Other algae grow to about 1 to 2 cm in length and stick to pond surfaces, waterfalls or rocks etc.

The trouble caused by algae in ponds can range from one of inconvenience to downright deadly.

Algae can create bright green totally opaque pond water caused by millions of algae cells congregating towards the surface of the pond. This condition is often referred to pea-soup water.

Algae grow inside pipes and block the pipes. This has been the cause of ponds being pumped dry. If the blockage occurs on the outlet from the gravity flow filter then the pump will continue to pump into the filter and overflow from the lid. It is therefore a good idea to always position a filter in such a way any overflow finds it way back into the pond.

Algae blooms in prolonged periods of hot weather can result in significant fish mortalities as a direct result of extracting all the dissolved oxygen from the water.

Pond Algae ...

It is not necessary to give the Latin names and species. What is important is the practical understanding of the situations you will encounter.

By way of general background it is important to realise that algae thrive on sunlight, warm weather and high nutrient levels in water. So if you have a problem it will possibly disappear in winter and be extremely concerning and annoying in mid summer. Algae problems have been getting worse over the years as a result of more nutrients (fertilisers for the algae, which are plants remember) finding their way into waterways and natural water supplies.

Because of the need for sunlight algae will tend to congregate at the surface as much as possible this is one reason why blanketweed forms dense floating mats.

Aquatic Plants and Algae

If a pond has plants then these plants will compete with algae for the available nutrients. In this way algae problems can be reduced but possibly not eliminated. For this reason if no other it is a good reason to introduce a range of aquatic plants into a pond or the circulating water system.

Fish eat plants so many pond keepers plant waterfall areas or even create a separate pond just to hold plants.

In ponds that contain only plants the problem of algae is normally much reduced for the reasons that nutrients from fish food are then eliminated.

Even in fishponds it makes sense to always underfeed fish to reduce algae problems fish can live quite easily on what is naturally available in most ponds so long as the pond is not over-populated.

Suspended Algae

First of all the usual problem of green water (sometimes brown) which does not go clear and even after changing the water the algae re-appears very quickly and this happens even if you have a filter installed.

Green/brown cloudy water is normally caused by millions of algae remaining suspended in the water. Each algae is about 4 microns in diameter (1 micron is 1 millionth of a metre or extremely small).

The reason why algae is not removed in many filters is due to its small size - if the medium in your filter has gaps/pores greater than 4 microns in size then the algae will just get re-circulated. By the way most filters suffer from this design problem so do not think it is a problem with yours. It is almost impossible to design to remove algae by filtration alone.

Algae growth is most common in ponds where conditions are most suited to their requirements - lots of fish being fed lots of food, warm water as it is in summer, and strong sunlight.

The waste products from fish eventually create nitrates in the water and most pond food contains phosphates and lots of nitrogen. These are fertilisers for plants, as you know from general gardening. Equally they are fertilisers for algae - remember algae are plants.

If there was no nitrate and phosphate there could be no algae.

Even if there are no fish in the pond it is possible to get algae problems. This is a common problem these days even in large waterways and is the result of nitrate contamination of groundwater from the widespread use of fertilisers in fields.

All tap water contains nitrates and in some areas probably phosphates. Also wind blows debris into ponds and water features and this debris also contains these substances.


Best in Class

Pond additives

Pond vacuums

Water features & ornaments

Pond Filters

Fish food

Pond lighting

Pond plants

Garden ponds

Pond pumps

Pond Tools


Polyex liner exclusive to Bradshaws

Complete Pond Kits

Pond Treatments

Oase Pondovac 3


Site Map | Bradshaws store items