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Types of Pond Algae 


Keeping Algae at bay while maintaining a balanced ecosystem is the ultimate goal for most pond owners. Yes, it is possible- provided you have good filtration, add the appropriate water treatments and have pond plants- but this process takes time to occur. First we need to understand a little bit about algae, then how to control it in order to decide upon which methods or approach you choose to take to care for your pond or water feature. It is all about Balancing the Nitrogen cycle!

 Algae are a varied group of simple, photosynthetic plant-like organisms that live throughout the world in diverse environments. Most live in either fresh or saltwater, ranging in size from single celled or microscopic (many may only be truly seen individually under the microscope) to several inches or even feet (kelp).  

 “Algae Blooms” is a term used when we experience an explosion of excessive algae growth which causes an unwanted abundance or “bloom”.  There are 5 main factors that cause algal blooms or increased algae growth:

1. Excessive Sunlight- provide some shade with structure or plants.

2. Abundance of Nutrients (decaying waste, fish waste, uneaten fish food)- Use sludge reducers and beneficial bacteria to reduce nutrient levels.

3. Slow Flow, Areas of stagnation. Increase aeration with aeration kits, spitters or pond pumps. 

4. Warm Water Conditions. Summer blooms are common. Water changes of 10-20% can help cool your water

5. Over-stocked Ponds (keep fish populations in check) or Over Fertilizing Pond Plants (only fertilize plants when needed- waterlilies and lotus need regular fertilizer for prolific blooms.

We avoid blooms in our ponds as much as possible as they can cause problems with water clarity and may also negatively affect your fish if they are getting caught in it or suffer from depleted dissolved oxygen levels.  Algae blooms may also affect how your pond runs- the pump and filters could get clogged up decreasing water flow. Make sure you always keep debris from restricting the pump intake.  The good news is that algae blooms happen fast, they don’t last forever and there are ways to break bloom cycles.   Learn more about the Nitrogen Cycle below.

Types of Algae– there are 3 main categories of Algae that live fresh water which are based upon how they grow in the water. You may experience them at some point throughout the season at various times of the year.


  1. Planktonic a.k.a. Single Celled Algae, are either free floating or in groups. These cause “green water” sometimes it can look brown, red or even yellowish. Often happens during warm weather. One of the most common types of algae- which can happen very fast and be really frustrating. Best control methods are with treatments-algaecides & beneficial bacteria; UV Clarifier or Sterilizer; and increase overall filtration (filter and/or increase amount pond plants, especially) floaters like Water Hyacinths & Lettuces.

Golden Gate Park


2. Filamentous- String or Carpet Algae. May be free floating in masses or attached to rocks or liner in groups forming long filaments. This is a large, diverse group of algae. String algae is commonly found in streams and on rocks. Carpet algae usually grows on the bottom of ponds on liner or rocks. Filamentous algae can happen in warm or cold weather, after a rainy period and in ponds high in mineral nutrients like Calcium or Phosphorous.

Natural Pond
Boat on Lake


3.  Macrophytic – plant-like and visible seen. The two most common are Chara and Nitella which look similar. These are more common in lakes or large farm ponds rather than back yard ponds. Algaecides should take care of these types if they become a nuisance.

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Controlling Algae

There is not one simple way to control algae forever. Continuous use of algaecides alone will kill algae, but then the dead algae will become fuel for the next algae bloom repeating the cycle. Using beneficial bacteria products a few days or a week after treating algae will help to break that cycle- eventually- after multiple treatments. You may even find that frequent use of beneficial bacteria treatments will reduce algaecide dependence. This may vary seasonally, what is best is to get to know your pond intimately. Keep in mind that every pond has its own unique environmental factors, so what works well for someone else may not work for you or even be necessary given your situation. Our goal is to strive for a balanced system, using multiple methods that balance each other. 

UV Clarifiers & Pressure Filters

UV Clarifiers and Pressure filter devices usually sit outside the pond and need to have water running through it so the bulb can "sterilize" the water killing algae. They can be used “inline” or added with an additional pump. The UV bulbs have a certain amount of “hours” that need to be replaced every year or so depending upon how long you run them. Read the manufacturers installation and care instructions. UV Clarifiers aka sterilizers- take care of “green water” only and will not help much with string algae. We have a great selection and several sizes of UV units, bulbs and quartz sleeves. Stop into our store so we can help. We also have pressurized filter units that have UV’s inside them making for the perfect combination. Check out our Pondmax Filters we have several sizes in stock.

Iongen- a gadget that helps prevent string algae. The controller is mounted outside the pond with the probe inside the pond in the skimmer or other area water freely flows 



Barley straw or Barley Extract- is an alternative method for “controlling” excessive algae growth. It is most effective when applied early in the year prior to the appearance of algae. Applied to cold water (less than 50°F), it may take 6-8 weeks to activate; warmer water (above 70°F) only 1-2 weeks. Barley straw remains effective for approximately 3-4 months after application. Barley straw does not kill existing algae, but it inhibits new algae growth. Barley straw, when exposed to sunlight and in the presence of oxygen, produces a chemical that inhibits algae growth. Use Barley / Peat Extract for faster results. 

  • Barley does not reduce the growth of other aquatic plants. In some cases aquatic plant growth has increased after barley applications because algae are no longer present to compete with the aquatic plants.

  • How often to Apply Barley: Spring and Summer (straw); Barley Extract use as Directed. Place bales at several locations around the pond and especially near streams & waterfalls. 

Pond Plants- Add a variety of pond plants to help shade your water (decreases sunlight absorption) and to help to out compete algae for nutrients. 

Oxy Pond Cleaner- is basically powdered hydrogen peroxide. Use as directed on the label- this product helps to dissolve debris and algae on contact. Works in fast and is used up in about 3 hours then it is "pet and people safe".  Avoid getting undiluted product on skin or plants.

Residential Garden


Algaecides preform better when you have algae, rather than solely as a preventative. Excess algaecides have been suspect to unanticipated fish deaths, even if the bottle says it is fish safe. Make sure you are home for several hours after initial treatment with an algaecide in case there is a problem.

There are many types of "algaecides" make sure to follow the directions on the label. Follow up with treatments of beneficial bacteria or sludge reducer to help keep debris levels low.

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