The Facts on Phosphate

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.

Pool Base Algae

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]

7 thoughts on “The Facts on Phosphate

  1. Hi,
    I am reading a lot about high phosphate levels in a pond and the results, but I cannot find a recommended figure of what phosphate levels should be. My pond has dropped from 0.25 to 0.00 after adding a blanket weed chemical and is nice and clear. Can you please advise me what is the correct phosphate level.

  2. Hallo Peter, I’d be interested to know which blanketweed chemical you used?

    Algae can continue to grow even at very low phosphate levels (0.005 mg/L), but will generally be less problematic when the level is below 0.035 mg/L (= ppm), and I’d aim to keep the level between these two figures in a wildlife pond. Unfortunately, not many test kits read well at these lower levels. In practice in ponds with many fish being fed, or tap water being regularly added, the level can rise much higher, and the 0.035 level is the more realistic one to aim for.

    The frothy blanketweed scum on pool surfaces is often worst in the late spring, especially after e.g. a warm April (like this year’s). In the warmer water, phosphate is released from the pool base sediments at the interface between the oxygenated water and the oxygen-lacking (anaerobic) sediments. Algae growing on the base bubbles up and floats to the surface on warm sunny days. Netting off and removing this algae can gradually capture some of the phosphate. As waterlilies and other waterplants get going later in May, they compete with the algae for nutrients and sunlight, and so can often help to reduce the scummy algae.

    Chemicals that aim to remove phosphate from the water should be used according to instruction, and not overdosed. If the phosphate levels are reduced to zero, any biological filter system will eventually fail to function properly, as the filter organisms require some phosphate.

  3. Hi,
    My established pond is 2.5′ in the ground and 2′ above ground. Contains
    6000 Ltrs. In volume .
    I have Two powerful filters running , second filter was added this year.
    Filter1 is 6000 Ltrs per hour
    Filter2 is 9000 Ltrs…
    The problem I have is besides blanket weed, which is now under control,however the Phosphate level is over 10. And the tap water is is around 8. What worries me is my 2ft Sturgeon recently on two separate occasions was floating upside down . Managed to revive him… My large Koi’s are fine as is other fish including Orfe.
    Forgot to mention I Very powerful air system pumping lots of Oxygen 24/7
    Can you advise

    • Hallo Nelson, If your Phosphate is over 10 mg/L that is very high, though that in itself is unlikely to directly affect fish. One way to keep such high levels down is to filter your tap water through a phosphate absorber. Your biological filters sound to be treating the pond at a good rate, though your sturgeon will outgrow that volume of water, and your koi may eventually feel the pinch. Regarding fish and chemicals, koi tend to be more tolerant than orfe, and orfe more tolerant than sturgeon. If you have used blanketweed treatments, these sometimes affect fish (especially sturgeon) as some contain heavy metals like copper and zinc (though they may not always say that on the label!). It is difficult to advise further unless you are in my local area, if so, contact me through the details on the main watergardensolutions website. James

  4. Hi, I have a ten year established pond. 14 ft x 16 x 4 ft deep. my nitrate and ammonia are fine. Ph is over 8 and phosphates are high. I tested my tap water and it is also high. Koi, cat fish, goldfish and tad poles seem to be ok, but water is not clear. Brownish green. Water is bacteria and barley treated, has lots of water lillies and water hyacinth too. Has an air pump, bio filter to water fall and another filter running. Don’t know what else to do to clear the water. Only good thing about the color of water is the birds of prey can’t see the fish, lol!

    • With quite a few fish (and no doubt some fish food too) it will be difficult to get your phosphate levels down, even if you have a lot of plant growth. I suspect that with the larger, messy fish like koi, you will have quite a bit of sediment on the base that will be swirling up to add to the brown tint. If you don’t already have a UV unit (or haven’t serviced the lamp recently), then try UV alongside your filtration, 30 watts or more. If all that algae dies back at once it will put a strain on the filters and oxygen levels, so cut back on feeding when the UV is turned on, and do some part water changes by siphoning or pumping sediments off the base, and topping up with fresh. If the pond has never been cleaned out, and you have a lot of sediment, you are likely to have to carry out a full clean at some point – get good local advice/help on that to minimise stress to your fish.

      • Not much sediment on bottom. Have a UV going. I think problem is town water. I tested it and it has high phosphate level. It is the only water source.

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