Green algae in my pond – Help!

Green water and pond algae can make a pond look like pea soup. It can be solved with a UVC.

Q: I have a pond of 1,500-gallons, with 14 koi of varying sizes. The water is pumped at 1,940-gallons per hour through a 3,000-gallon Mag-nit onto a UV 15W and then into a Cyprio Green Machine 4,000.

The problem I encounter every year is that from late Spring through to Summer, whenever the sun warms up and shines on the pond, the water clouds over giving a horrible brown and dirty effect, but by the morning the water is clear until the sun comes out again.

I have no plant life in the pond as my koi just eat them and when I test my water it seems fine. All my fish are healthy and grow at a steady rate year on year.

Please can you help with my problem so I can enjoy looking at my fish all year round.

Thanks for the details about your pond set up and the frustrating intermittent clarity problem that you are experiencing in the warmer and sunnier months. From what you describe, your koi are showing no signs of distress for this summer phenomenon, as you say they are growing well and that the water quality parameters when tested are fine.

This suggests that the biological function of your pond and filter is functioning well, breaking down to soluble (and invisible) pollutants that are released by your growing and healthy koi at the same rate at which they are produced. We can safely conclude that your koi’s health, (even with your current concern over water clarity), is likely to remain unaffected.

However, this is of little consequence as the visual appeal of your pond and koi are spoiled daily by the periodic cloudiness that makes your water (and koi) appear dirty. I am immediately drawn to the regularity of your clouding phenomenon which occurs during the daytime and am lead to suggest that it is caused by a daily bloom of algae.

Your pond is well stocked with growing koi that I assume are being fed regularly, leading inevitably to an accumulation of phosphates and nitrates in the pond that is likely to go on and fuel algae growth. In a planted garden pond, where growth of aquatic plants continuously draws on the available nutrients in pond water, there is less tendency for algae to bloom.

Yet your plant-free koi pond will be less effective at reducing the accumulation of nutrients in the water, making them readily available for the use by nuisance algae. As you have a Mag-nit installed in line between your pump and filter, I guess that blanket weed growth is minimal, allowing the greenwater-causing unicellular algae unhindered growth in your pond. Although your complaint is one of ‘brown and dirty’ water, this is still likely to be caused by a type of algae.

From your letter, it is clear at you have a 15W UV installed to control greenwater and yet it still appears not to be having the desired effect when it is put to the test in the warmer sunnier months. There are two or three areas that you should look into when trying to solve this problem.

1. UV maintenance. The expected working life of a UV bulb will vary from six months to a full year depending on the manufacturer. At night, you are likely to see the bulb glowing inside your UV unit, but if that bulb is beyond its working life then it’s output in the germicidal UV wavelengths will not be sufficient to eradicate the algae. An illuminated bulb does not necessarily mean that it is an effective bulb. If you have not changed it since last year, buy and fit a replacement bulb; it should cost about 10.

In the same way, check that your quartz sleeve (the transparent tubular barrier between the UV bulb and the flowing pond water) is clear and free from line scale, especially if it has taken out to use an over the winter. Any build-up of material inside the quartz sleeve will also reduce the effectiveness of the UV bulb.

2. Flow rates through the UV unit. There appears to be something a little unusual about the facts and figures that you have supplied about your pond, in that they appear to be mismatched. Your pond’s volume is 1500 gallons and yet you’re reportedly turning over nearly 2000 gallons per hour through your system. This is an incredibly high turnover rate for a 1500 gallon pond, where a flow rate of 800 gallons per hour would be sufficient.

Furthermore, even though a Mag-nit will handle up to 2500 gallons per hour (at one and a half inch tubing) there is no way this same flow rate will continue through the constrictions of a UV unit. A typical 15W UVc rated at keeping a 2500 gallon pond free from algae will at best handle a flow rate of approximately 1000 gallons per hour. Your system then goes on to feed a Green Machine 4000 that can handle a higher flow rate of 2500 gallons per hour.

So there is either an incredibly high turnover of water being pushed through your UV (the maximum possible for that unit) by your pump or you are rating your pump’s performance on it’s book/box reported flow rate. This is likely to be given for your pump’s performance at zero head with no pipework restrictions, where in fact you are pumping through a series of constricting units, situated at a nominal vertical distance above your pond.

As you know, the flow rate of your pump will be greatly reduced by the height above the water surface it has to pump, and the diameter of your system’s pipework. If your pump is over engineered for the pond’s requirements, besides having unnecessarily high running costs, there is a risk that water could be flowing through your UV unit too rapidly, reducing its effect on clearing greenwater.

UVs clear greenwater by irradiating the algae cells suspended in the water, causing them to stick together into larger more filterable clumps. If the flow rate is too rapid in relation to the power of your UV lamp and therefore the contact time is reduced, then it’s effectiveness at clearing algae will also be reduced.

Currently your UV unit appears to be bordering on being ineffective, as it struggles to cope with the proliferation of algae cells in your pond under favourable growing conditions in the warm, sunny daytime. At night, your UV and filter system is able to catch up when algae growth and cell division will be greatly reduced.

Remember, your 15W UV is rated at clearing a 2500 gallon pond, so it should easily cope with your 1500 gallon pond’s workload. Check what your turnover rate is after your Mag-nit. You can do this by timing how long the flow rate takes to fill up a large vessel of known volume. (This can then be used to calculate your hourly turnover rate.) If it is excessively high for your pond, reduce it to about 800 gallons per hour maximum.

By reducing your flow rate (and hence improving your contact time within the UV) and checking the working condition of your UV bulb and quartz sleeve are satisfactory, you should see an improvement in your pond’s water clarity throughout the day and never witness that cloudiness again.

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