Algae Control 101

The two most common forms of algae that ponds experience are planktonic algae and filamentous algae. Planktonic algae is the type that gives pond that distinctive pea green, soupy color. Planktonic algae in moderation is very important as is the base of the food chain in a pond. However, it is when it is in excess that you get the green water.

Filamentous algae is what you typically see growing on rocks and floating on the surface in mats. It is the stringy, hairlike algae that you often see in the spring and summer months. It has little redeeming value.

Algae in moderation is good for a pond, but when it becomes over abundant, it can cause severe water quality problems. Algae consumes oxygen at night to continue its life cycle, so when there is a severe outbreak, large amounts of oxygen are consumed and can seriously deplete the dissolved oxygen level in a pond to a point that cause fish kills. Not to mention that fact this its flat out unsightly.

Algae blooms are caused by three factors: excess nutrients, too much sunlight and low oxygen levels.

Nutrients come from a variety of sources, most notable fish and animal waste, fish food and dead/decaying vegetation. Other sources would include fertilizer run-off and run-off from nearby farms and pastures. All of these sources provide much needed nitrogen and phosphorus that algae need to bloom and grow.

Ponds that sit in direct sunlight or have very few aquatic plants are also at risk of suffering from algae problems. This is why generally the most and worst algae problems occur in the summer when the days are longest.

Finally, ponds that experience poor oxygen levels will also suffer from algae control issues. When there are high levels of waste in ponds that are stagnant and still, oxygen levels can be depleted due to the amount of oxygen needed to break down the waste. Algae blooms come and go, but when there is little dissolved oxygen in the water, dying algae will feed new cycles of living algae because there is not enough oxygen to break down the dead vegetation before it can fuel more growth.

First of all, algae should not be looked upon as a problem. It is merely a symptom and a sign that there are other problems and that your pond is out of balance. What it means is that one or more of the causes above are the true problems.

A short term, quick fix is to use an algaecide. However this is just a short term solution because when you simply kill the algae and leave it to degrade on its own, it will just fuel re-growth. A quick kill can also cause an oxygen depletion, which can stress or even kill fish.

The ideal approach for using an algaecide for quick algae control would be to use the algaecide as a pre-cursor to the long-term approach discussed below.

A more sound and lasting approach would be to control the causes of the algae growth – nutrients, sunlight and oxygen.

Nutrient control involves limiting the amount of nutrients that enter that water that can feed algae. This can range from manually skimming debris and waste from the pond, physically removing the algae, installing an effective filtration system or limiting the number of aquatic inhabitants, but it should always involve the use of beneficial pond bacteria. Pond bacteria is crucial in breaking down and degrading all types of organic waste and should be used regularly.

Reducing sunlight penetration is a more difficult because you only have so much control over that, but a good pond dye and even the addition of aquatic plants can help to shade the water and limit the sun’s effect. A good rule of thumb would be 60%-70% coverage with plants.

Improving the oxygen levels in a pond can be done with a pond aerator or even a pond fountain. The infusion of oxygen will invigorate pond bacteria, making them more efficient at their job, while also keeping the pond water circulating (another deterrent to algae growth).

Effective algae control begins with knowledge and understanding of the type of algae you have and what causes the algae to grow. Once you have armed yourself with the proper information, treating the causes and controlling the sources become more manageable. Then, year after year, preventative maintenance becomes second nature and severe algae blooms become less and less of a concern.

For more details, visit this other Algae Control article.

Great information - was very helpful

I have controlled string algae all summer. Left for several weeks and now my pond has been basically taken over by the string green meanies. We are on the brink of cold, I mean, really cold weather. Can I winterize and hope this stuff is frozen and killed until next spring? Or do I need to get in under control before the long, cold winter so it isn't worst than ever next spring? Thanks in advance!

String algae comes and goes with the season. I control mine when it gets thick by pulling it. I have never put chemicals in my ponds in the 40 years I have had them. The only exception is dechlor.

Beware of all these quick fixes, they may have long term effects.

Thanks for the advice. I can stay on top of it next season. So basically, I can let the mountain temperatures take care of it soon? We have a season from June until October. It can't really hurt anything, can it?
Thanks for all of your help!

My adult Koi love it. They like to force their way through it for fun. I think they also eat some and they used it for spawining. String algae does not make pea soup, that is another type of algae.

We have had our pond for five years now. We have experienced planktonic alge and filamentous or string alge but this year, for the first time, we have been taken over by a dark green, beard-like alge that does not seem to cling to anything but just covers the bottom and multiplies.

We have two ponds that cascade down to a third where our Koi live. The alge has taken over the top two leaving me to believe that our koi are eating what ends up in the bottom pond.

We cleaned out the top two completely and added barley extract per the instructions but it seemed to have no effect. The alge was back within 4 weeks. This is the first year we have seen this and honesty do not know what to do.

Any thoughts?

Ronald - I get a thin film of algae that covers much of the rocked bottom and sides. I use GreenClear every 2 or 3 weeks to keep it in check. Since yours also covers the bottom, that might work.

Oh, this is no thin film...........

This is handfuls of green cotton-like alge completely covering the bottom of those ponds without fish. We have had string alge before and this is very different.

Sorry, I am not familiar (thank goodness). You could probably still try Green Clear.