Do you have persistent problems with blanketweed or duckweed in your pond? Phosphate (a form of phosphorus) found in fish food and tap water may be the problem.
Phosphorus is an essential element for life. Usually it is bound with oxygen and other minerals to form phosphates. It is also found in animal bones (calcium phosphate).
High levels of phosphate in ponds are a major cause of green water and blanketweed problems, and can also cause nuisance levels of duckweed.
You may have heard of phosphate as one of the main ingredients of garden fertilisers (“NPK” = Nitrogen, Phosphorus & Potassium). In ponds there are usually ample amounts of nitrogen in the water, but if phosphorus/phosphate levels are kept low then algae growth will be limited.
In balanced ponds and lakes there is enough phosphate for moderate growth of waterplants but not so much as to cause excessive algae growth.
Where do phosphates in my pond come from?
Some comes from the soil in planting containers, but commercial aquatic soils do not contain high amounts. Aquatic fertilisers are generally only needed for waterlilies to help them to flower better. Usually such fertilisers incorporate slow release phosphate applied in pellets close to the roots, to limit any leaching into the pondwater.
The tap water used to fill the pond is a major source. Phosphates are not only found in the rivers used for water supplies, but in the past twenty years phosphates have been deliberately added to tap water to reduce pipe corrosion**.
The other major source is fish food which contains some phosphorus as it is an essential part of the diet. A large proportion will pass through and be released in the fish waste. This waste releases phosphates into the water, and also stores phosphates within the sediments on the pool base.
Run-off from lawns, borders, and surrounding paving can wash in phosphates, especially if the garden has had fertilisers applied, or if there are many bird droppings on the paving.
What can I do to prevent high levels of phosphates in my pond?
Top up with rainwater. Rainwater is generally safe to use and if it runs via a storage tank, most debris will be settled out. However, ponds with higher stocks of fish should also be occasionally filled with tap water (using a dechlorinating treatment), as tap water can add some essential hardness* to the water (*minerals which are missing from rain water).
Do not overfeed fish. Use good quality fish foods, and only feed high-protein ‘growth’ formulas during the summer months. Some fish foods list the phosphorus levels on the label. Ideally foods should have 1% or less; poorly formulated foods can contain more than double this amount.
Design the pool edges to prevent run-off from washing in.
Have sufficient pond plants. As well as being decorative and encouraging wildlife, pond plants such as waterlilies, submerged plants, floating plants, irises and other marginal plants can all use up the phosphates in the water and help the pond to reach a better balance. Remove old and damaged leaves as the season continues.
If you have a filter system, clean out the captured debris regularly. Maintaining a filter in this way can remove those phosphates that are bound to fine particles in the water. This can almost halve the phosphate levels in the pond.
Do tapwater and fish food really make that much difference?
In a small back-garden pond of 500 gallons with goldfish. If you were to:
– feed a small handful of a typical fish food, or
– top up the pond with 1” (25mm) of tap water,
either action would increase your pond phosphate levels by an amount (0.1 mg/Litre) sufficient to encourage some algae growth. This might be the green scummy algae that grows around the pond edges, or on the pond base.
If I stop phosphate getting into my pond, will that solve algae problems?
It will certainly help, however, the sediments on the pool base may continue to release their stores of phosphate for some time. If you have deep sediments a pool clean may be required, but there are other management and treatment approaches that can help.
We can suggest a plan tailored to your specific situation. We will carry out a free water test for phosphates alongside any other paid maintenance visit – just mention this article when you contact us for this special offer.
**Stop at source?
Water suppliers in the UK are adding thousands of tons of phosphates to our tap water every year, in principle to reduce corrosion of old lead pipes [external article]. However, that phosphate is causing problems not only in our ponds, but also when it leaks into the environment from water pipes. It is also costly to remove from the waste water that eventually reaches our sewage works. The long term solution is for water suppliers to replace or reline old lead pipes – why not prompt your supplier to do this?
[Updated January 2018]