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I have a small pond (I don't really know the volume, maybe 400 liters) with goldfish inside, they seem healthy with no apparent problem.

The pond is constantly invaded by micro-algae that cannot be filtered, the water is constantly green and I can see about 5 cm deep from the surface.

From what I've read it doesn't really bother the fish and they actually like it, but I have no easy way to check the health and number of fish in the pond if I can't see below the surface.

I've tried using a product for killing off algae and a UV filter, which worked for a bit, but they seem to have no effect now (my suspicion is that the UV filter might be dirty).

Also, the quantity of algae varies quite a bit based on the season, it once got almost totally clear (strangely without using any special product), but I wasn't able to replicate that result.

I tried also adding some plants, but the fish (and an hailstorm for the floating ones) destroyed them.

If that can help, I live in northern Italy.

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First of all you will have to be sure you have good filtration, your pump and filter needs to filter all the water at least once every hour.

The UV bulb needs to be changed every 6 months of use as the UV light output will degrade over time. The glass of your UV lamp needs to be cleaned when you can see it is dirty; you can use vinegar if there is limescale on the glass.

UV light is one of the best ways to get crystal clear water in a pond, but you need to be sure you have the right waterflow for it to be effective. All the water in your pond needs to go through the UV sterilizer no less than once every two hours.

A plant filter can be a good way to limit the amount of algae in your water, but it needs to be fairly large to be able to deprive the algae of the nutrients in your water.

If you have an area in your pond where your fish cannot reach, you can plant watercress; it is one of the best plants for removal of nutrients from your water and it grows quickly. Watercress is edible and is one of the plants with the highest nutritional value.

Partially shading the pond can help in getting less algae, but a pond should be visible, so this might not be an alternative for you.

If you have sand on the bottom of your pond you can get freshwater mussels - they will filter out the algae, but you are dependent on keeping the water temperature below 25 °C to keep them alive. One single mussel can clean a 100l fish tank from green water in about 6 hours.

More information about mentioned mussels could be found in a relevant article on Wikipedia.

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    I can probably build some sort of second container with watercress inside and pass water through that before reentering the pond. I like the mussels idea but i'm not sure where to get them. Also the pond will get quite chilly in the winter (the surface will freeze), can the mussels survive that? I also like the "natural" method better, without the uv filter – Yeeter Jul 27 at 14:30
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    @Yeeter if you do you are half way in building a plant filter,you can combine it with a small waterfall or a little stream.the mussels can be found in north europe or north america but getting them from nature carry some risk of parasites. – trond hansen Jul 27 at 14:36
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    mussels need to be protected against frost but they can handle temperature down to about 2C. – trond hansen Jul 27 at 14:48
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    Nice, i think the temperature shouldn't be a problem. I'll try to get some mussels in some way. If that can help i added where i live in the question. – Yeeter Jul 27 at 14:57
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    some garden centres or petshops have mussels you can use in garden ponds,the problem with green water is common in ponds all over the world. – trond hansen Jul 27 at 15:06
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You are probably over-feeding, putting too much nutrients in the pond. I have a few ponds, never an algae problem, sometimes algae in the first month as they mature. I think algaecide is just the wrong thing, it kills regular plants in my experience. Feed less and when algaecide is gone, put in plants, they will out-compete the algae for nutrients. Duckweed is an easy start, depending on what is available, hornwort, Anacharis, Cabomba, maybe rooted plants like Vallisneria. Goldfish/carp will eat some like duckweed. You can tell if there are carp in a pond by the duckweed; if there is a lot of duckweed there are no carp.

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  • I also thought about nutrient imbalance, but i don't think i feed them too much. There likely is some plant matter to the bottom (which i can't see) that might be releasing nutrients for the algae. Unfortunately those goldfish seem to play and eventually destroy any submerged plant i put in. I can confirm that there is no carp – Yeeter Jul 27 at 15:01
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    Goldfish are a part of the family of carp. So yes, there are carp in your pond ;) – Allerleirauh Aug 2 at 21:07

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