I Have A UV Light In My Pond But Still Have Green Water

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Written By Mark Washburn

Mark has 20 years of experience as a professional pond management consultant.

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John writes, “I have a 1500 gallon backyard pond and have installed a uv light in it which was supposed to help with green water problems. It doesn’t seem to be helping very much, and I wondered if I did something wrong?”

Answer: Hi John and thanks for the question. Your plan to use a uv sterilizer in your pond to combat green water is a good idea. In truth, when you hear companies talk about fighting pond algae problems with a uv filter, it’s single cell, or green water algae that they are talking about specifically.

In contrast, string algae won’t be affected because it can’t pass through the tube and be exposed to the ultra violet light, which does all of the damage. In the case of green water, this discoloration can vary in a pond and the degree of just how green and dense it will get is totally dependent on how many little, single cell algae you have growing in the pond. These can expand in number quickly when conditions are right so a pond can be pretty clear or lightly tinted one day and totally green the next.

A pond uv light basically irradiates these little critters and will either damage or kill them and in doing so, it makes them clump together in a larger mass which is more easily filtered out of the water by a good quality pond filter. What’s interesting is that individually, these single cell algae can be so small that they’ll pass right through many filter systems.

Which brings me to an important point. A UV system is not the only required component in a small pond’s filtration system. Ideally they should be paired with a good biofilter which actually captures and breaks down these dead or damaged algae cells. UV will do nothing to reduce nitrites, nitrates, or other material which can be broadly classified as nutrients coming from fish waste, among other things. Only a biofilter will help with these so it’s important to include this in the mix.

Green Water Continues

Interestingly, many people who have UV light installed in their ponds may still have problems with green water. This usually indicates that something will need to be adjusted to get this to work better. Most manufacturers have ratings for how much water flow should run through the uv over a period of time. For example, a very common rate would be the full gallon volume of the pond should circulate through the uv and filter system once every hour. This insures that the water and algae cells are passing in front of the uv light frequently enough to be affected.

It’s best to follow the product maker’s suggestions but if you need to, it may also be useful to slow the water flow rate down a little bit if you’re still having problems. A slower pass rate may help expose the algae to the light a bit longer and this can make a difference in performance. Normally some type of restriction valve can be put inline and help with this flow rate control.

You may find that some uv lights come with several suggest flow settings. A higher or faster rate would allow the unit to work as a clarifier, which means it may control algae, but not decimate things like beneficial pond bacteria so much. UV is indiscriminate in what it will kill or control…good bugs, bad microbes, viruses, and algae are all targeted.

On the other side of the flow rate coin, a slower flow rate will provide a sterilizer effect on the water. A more thorough eradication of all affected elements will be made because of the longer exposure time to the ultra violet light. Either rate, or somewhere in between will likely create the best results for your pond in terms of algae control. In an ideal setting, you would want more a clarifier effect but this may not be possible for every pond.

Another reason why uv light may not be getting the job done is that the uv bulb will need to be replaced from time to time. Many people assume that because it’s still lighting up that it’s working fine but this isn’t always the case. Most bulbs have a operational life where they will provide ample ultra violet radiation and then begin to drop off in terms of performance. Twelve to fourteen months of operational life is common. After that you’ll likely want to replace them for improved performance.

And finally, if you’re in the market for a uv light or considering it for your pond, it’s important to make sure you get one that’s rated for your pond’s gallon size. A pond owner really shouldn’t scrimp on any of the important mechanical components in a pond, such as the pump or filter system. UV light is no different. Remember that to work well, the full gallon volume of the pond should probably pass through the uv pond filter at least once per hour, all day long.

With a bit of troubleshooting and detective work, you can get better performance out of your pond uv light and in doing so, you should be able to control green water algae without the use of chemical algaecides.

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