How to reduce excess micro-algae in pond

Question

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.

Answer 1

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.

Answer 2

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.