How to Control Winter Pond Algae

Winter Pond Fish CareMost things in your pond go dormant in winter. So why isn't the same true of algae?

While pond algae is usually less active in cold months, both string algae and single-cell algae make the occasional winter cameo.

Luckily, you can take some easy steps to combat it - if you decide it's worth dealing with at all.


AT A GLANCE: How to Control Winter Pond Algae

  • Algae will not usually harm fish or plants, so you can leave it alone until spring.
  • Contact algaecides can kill string algae.
  • Following good winter care practice - like aerating your pond and adding Cold Water Beneficial Bacteria - will keep your finned friends happy and healthy.
  • Use Beneficial Bacteria, plants and good filtration to keep algae away in warm weather.

Does Pond Algae Die in Winter?

Winter koiPond algae usually has a difficult time thriving in cold weather. The nutrients it needs to survive - usually from organic material like fish waste and fish food - are scarce. The water is cold. And snow prevents the sunlight it needs from reaching the pond.

Certain factors, though, can bring green to your otherwise dormant pond.

Fluctuating Water Temperatures

Water doesn't hold heat the same way air does. Shallow water will heat up quickly on a sunny winter day, while deep water will maintain a relatively consistent - and warmish - temperature, even on the coldest days.

Water's weird way of hanging on to heat could make your pond just warm enough to support algae growth.

Sleeping Algae-Fighters

Algae survive on excess nutrients in your pond. For single-cell algae (green water), that main nutrient is nitrites. For string algae, the main nutrient is nitrates.

In the warm months, the best way to prevent either type of algae is to starve it. Beneficial bacteria will eat nitrites before the single-cell algae has a chance to grab a hold of it, and plants will consume nitrates.

In winter, your plants are dormant and bacteria aren't as efficient as they are in summer. That means any organic material that does make its way into your pond has a better chance of becoming algae chow.

More Sunlight than You Might Expect

A thick blanket of snow will prevent sunlight from reaching your water. A layer of clear ice will not.

Algae is an opportunistic little thing, so it will take advantage of any bit of sunlight it gets.

3 Ways to Deal with Winter Pond Algae

So you have a winter algae outbreak. Don't worry. Fixing the problem is easy - if you decide it's even a problem at all.

1. Wait It Out

Algae usually poses no risk to your fish or plants. This fact stands true whether it shows up in the heat of summer or the dead of winter.

Algae can even help your pond by giving your fish something to snack on during warm days and adding much-needed oxygen into the water.

Leaving the algae alone is usually the best way to handle winter outbreaks. Once the mercury rises, your algae-fighting plants and bacteria will spring back into action.

2. Fight String Algae with a Contact Algaecide

Pond algae generally falls into one of two categories: string algae and single-cell algae. Single-cell algae is the microscopic stuff that turns your water pea-soup green. String algae is the stringy gunk that clings to your rocks and waterfalls.

If you have the latter, you can treat it with a contact algaecide or similar product. If you don't have any on-hand, you can find it at our Online Store:

EcoBlast (Contact Algaecide)

S.A.B. (Concentrated Bacteria & Enzymes) 

Contact algaecide is a powder you sprinkle directly on to the algae to kill it almost instantly. As it dies, it releases from your rocks, giving you the chance to easily scoop it out of the pond.

Remember: Dead algae becomes food for new algae, so you have to remove as much of the debris from the pond as possible after treatment. Contact algaecide also won't prevent new algae from growing, so make sure you add plenty of plants to your pond come spring to pull the algae-feeding nutrients out of your water.

One final note on algaecide: Contact algaecide is different from liquid algaecide. Many liquid algaecides don't work in cold water, and we don't recommend using them even in the warm months.

3. Keep Your Winter Pond Healthy

Ecosystem Ponds are easy to care for and beautiful to look at any time of year.

Here are some tips for keeping everything running smoothly in the cold months:

Add regular doses of Cold Water Beneficial Bacteria to keep your water healthy for fish and help break down organic debris. This bacteria is specially formulated to work when water is too cold for regular bacteria.

Buy Cold Water Bacteria

Aerate your pond. The best way to do this is by adding an aerator. A running waterfall - if you choose to keep yours turned on for the season - can also help.

Browse Aeration Kits

Don't let the pond freeze over entirely. While an aerator will help keep a hole open in the ice, you can also add a deicer for extra protection. If the pond does freeze over, place a deicer or hot pot of water on top of the ice to gently melt a hole for gas exchange.

Buy a Pond Deicer

Don't feed fish when water temperatures are below 55 degrees. Fish enter a semi-dormant state called torpor in the cold months. Any food you add to cold water will either go undigested in their bellies - bad for the fish - or sink to the bottom of the pond - bad for your water quality.

Buy a Submersible Thermometer

Getting Ready for Spring

Most winter algae problems will work themselves out. If you still find yourself battling green water or string algae in late spring, however, you might need to take additional steps.

Warm-weather algae control starts with adding lots of plants to your pond and regular doses of Beneficial Bacteria. If nothing seems to work, you might need to upgrade your filtration.

Need more help? Check out our Ultimate Guide to Clear Water.

More Winter Pond Care Tips

How Do Pond Turtles Survive the Winter?

How to Winterize an Autodoser

Dealing with Spring String Algae

How to Make a Hole in Pond Ice

How to Winterize a Pond

How to Protect Your Pond from Extreme Cold

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