Dealing with Algae in Koi Ponds


DIY and how-to

Pink and white flower petals floating on the surface of a koi pond with fish visible beneath.

Dealing with Algae in Koi Ponds

Algae isn’t as bad as you may think. It is a plant that puts oxygen into the water during the day and takes it out again at night. Green water is caused by really small algae – tiny single-celled plants. They replicate at lightning speed, causing ponds to go slightly off colour – often a light grey, before becoming a light browny green and then suddenly, completely green – almost overnight.

The green colour prevents you from seeing your fish, but the fish don’t really care. Koi are carp and they are very comfortable in green murky water – just like the natural river and dam systems that are their normal living environments. The only cause of concern with green water is low oxygen levels overnight and the fact that if something is wrong with your Koi you won’t see it. Because we like to see our Koi, most Koi keepers nuke these algae cells with UV lights. Remember that even if a UV lamp is glowing purple, it does not mean that the globe is still emitting sufficient UV to kill off the algae cells. You should replace the globe once every 8 to 12 months.

Then of course you get the real culprit – blanket weed algae. It grows up to a metre a day and clogs filters and pumps. No UV is going to help you with this monstrous pond invader and the only solution is to scrape it off the sides with a wire brush. Let it filter through your bottom drain and catch it all from your settlement chamber before it gets into the filters – or else you will have to clean those as well.