Blue-Green Algae in your Pond: Warning Signs and Solutions

This article discusses the dangers of blue-green algae in your pond, water garden, river, lake, and other waterways. Warning signs of blue-green algae and common causes, as well as solutions for pond algae control.

What exactly is Blue-Green Algae?

Algal bloom at Grand Lake St. Mary's, Ohio, 2010. Photo by Russ Gibson, Ohio EPA

Algal bloom at Grand Lake St. Mary’s, Ohio, 2010. Photo by Russ Gibson, Ohio EPA

Ironically, blue-green algae isn’t even technically algae at all; the term actually refers to cyanobacteria. Being that cyan refers to a blue-green color, and cyanobacteria very much resembles common algae, the name blue-green algae is the commonly used term.

While similar in appearance, algae and blue-green algae are completely different organisms. The difference: blue-greens are very primitive organisms that are not really algae. They photosynthesize like algae, but they are actually bacteria. Thus, blue-green algae has much more potential to cause harm to an aquatic ecosystem (such as a pond, water garden, golf course waterway, etc.) and even the surrounding people and animals.

How is Blue-Green Algae Dangerous to my Pond and Pets?

While not every blue-green algae bloom produces toxins, numerous studies have shown contact with the bacteria can be harmful. Recent studies have shown the probability that an individual bloom containing Anabaena, Microcystis, and/or Aphanizomenon (common problem suspended algae) will be toxic is greater than previously thought (45-75%).

Since cyanobacteria toxins can be lethal to pets in relatively small amounts, caution should always be taken when a bloom occurs.

pet blue green algae toxicityWarning signs of blue-green algae toxicity are: dead fish and/or waterfowl, unexplained sickness/death of a cat or dog, unpleasantly scented water, and skin rashes following human contact with water. Furthermore, one should be cautious in observing pet behavior if worried about the toxicity of an algae bloom.

Common side effects of cyanobacteria poisoning in pets: vomiting, diarrhea, jaundice, seizures, disorientation, coma, shock, excessive salivation, shortness of breath, and death.  With these consequences looming, it is important to be educated and proactive in the fight against blue-green algae.

Learn More About Blue-Green Algae and Dangerous Toxic Algae to Your Pet

How Does it Affect my Pond?

Unlike other types of algae or string algae growth, blue-green algae forms a blanket of slime on the surface of a body of water.  If this layer of bacteria grows enough, it can completely block out sunlight from the depths of the water.  In this way, it can thwart growth of competing algae, and completely dominate a body of water.

If left untreated, blue-green algae can very quickly cover the entire area in a thick, green layer of bacteria.

Facts on Harmful Algal Blooms (HABs) from the Center for Disease Control

  • Dense CyanoHABs (Cyanobacterial harmful algal blooms) can block sunlight and use up all the oxygen in the water, killing other plants and animals.
  • Some cyanobacteria that can form CyanoHABs produce toxins that are among the most powerful natural poisons known.  These toxins have no known antidotes.
  • CyanoHABs can make people, their pets, and other animals sick. Often, the first sign that an HAB exists is a sick dog that has been swimming in an algae-filled pond.
  • Children are at higher risk than adults for illness from CyanoHABs because they weigh less and can get a relatively larger dose of toxin.

In some cases, blooms of blue-green algae have been known to appear seemingly overnight.  This is a result of blue-green algae’s ability to form at virtually any depth of a water body.  These blooms growing at a larger depth can be difficult to identify, creating problematic situations for inexperienced pond keepers.

What can I do to Prevent Blue-Green Algae?

Preventative measures for blue-green algae are similar to those of common plant algae and provide the first line of defense. The main cause of cyanobacterial blooms is the accumulation of excess nutrients in a body of water. The following measures can help prevent a blue-green algae bloom from occurring:

  • Do not overfeed fish or birds; the excess can cause nutrient buildup.
  • Plant and maintain native flora around ponds and streams, which help filter water and don’t require fertilizer.
  • If applicable, properly maintain your septic system.
  • Do not overuse lawn and garden fertilizers, especially near bodies of water.
  • Do not allow pets or livestock to defecate in streams or lakes.

For maintaining a fish pond, learn more ideas in our article on koi pond care in the summer.

How to Protect your Fish, Pets, and Ecosystem!

  • After identifying an algal bloom, keep animals and children away from the pond until it is diagnosed or removed.
  • Scoop out all excess debris and start the damage control process.
  • Use a liquid additive that will balance your pond’s ecosystem while removing blue-green algae.
  • Try not to use an algaecide as these can be dangerous to fish and wildlife.

Choosing a natural solution like Nualgi Ponds will help curtail both the causes and effects of blue-green algae while increasing dissolved oxygen and providing live feed for fish. For ongoing/preventative measures, the 12 essential nutrients in Nualgi Ponds will balance the Nitrogen, Phosphorous and Carbon Dioxide levels to create a more friendly environment for fish and plant health.

Try Nualgi Ponds in Your Pond!

Why not save some stress and stop the problem before it begins? We’re confident that our breakthrough supplement for controlling algae in a pond will help you in the fight against blue-green algae, and we would love to join the battle on your side!

Try Nualgi Ponds Today

pond blue green algae control


  1. I have a fairly large pond. . . 30 by 30 i guess. . . I live in the country, and the pond is pretty much natural, except for the liner. . .there are a few gold and koi fish.. .
    I have a water fall that runs year around because it freezes here, so it is to control mosquitoes and oxygen for the fish. .
    The water fall was having some problems so I unplugged it, and turned my 60 ft well hose with a sprinkler on to keep things running till I could get to the repair. Walked away and forgot it over night. When I remembered it the following morning, it had turned Turquoise and there was a film on top of the water. I took the sprinkler out from the bottom of the pond, and sprayed the top layer to the run off area. . . Then I did a well water test. No problems? There are many marijuana grow rooms in my neighborhood and they dump their fertilizer into the ground. . What do you think?

    • Turquoise is generally the color of blue green algae. Nualgi will treat that effectively. You can email pics to for further diagnosis.

  2. Hello,
    I have a small pond that I THINK is about 150 gallons. Do you have a suggestion about how I dose .1 of a ml? Would it be a certain number of drops from the dropper?
    Thank you,
    Vicki Sawyer

    • 3 drops a week

  3. How do I calculate the volume of my pond, to work out the dosage, as we acquired it 2nd hand and never saw the specifications when it was bought. It is roughly 8’ft by 4’ ft but it is half way between a comma and a kidney (being waisted in the middle) shaped. It also has a shelf about 18” from the top that varies from 4 ½ “ wide most of the way round, to approximately 18” at the narrow end, so that beyond the shelf the shape is fully kidney shaped, where it then drops about another 18”’ in depth before reaching the bottom.
    For the first time in over 20 years it has been in existence, last year we had a severe problem with Algae bloom. It was summer at the time and warm enough yet wet enough to use a series of paddling pools to collect rain water in, to give a natural source of water to transfer all the fish and newts into. We cleared out all silt & scrapped the plants, purchasing a whole load of new oxygenators, before adding back the paddling pool water, fish & newts. The 2 paddling pools of water filled roughly 2/3rds of the pond, but everything was getting quite distressed, so we then had to risk filling the rest with the hose – at least it added much needed oxygen!
    So again we had true measure of what liquid went back. It did recover but it took several weeks, reaching its best just before autumn. But over winter (although I can’t see much actual algae) the resulting green water has came back. Despite the cold I removing and replaced about 1/3 of the water for 2 weeks, then last weekend removed all the water down to the shelf, but it does not seem enough, the water is still not clear there are still flecks of green and I can only see down around 9” before everything becomes obscured. So this time I want to try treatment, but can’t hazard a guess as to the dosage. Can you help?

    • Lauren, an approximate volume calculation is good enough for Nualgi dosage purposes. A little over dosing does not do anything negative. Given your situation, I would strongly recommend filing a troubleshooting request using this link…
      Your pond should be around 1000 gallons.

  4. What types of plants are recommended for filtration. Zone 4 state of MN usa

    • Thanks for your question Lisa. The following plants are a good start:

      Iris versicolor, Blue Flag Iris
      Pontederia cordata, Pickerel Rush
      Sagittaria latifolia, Arrowhead
      Hibiscus moschuetos, Swamp Hibiscus
      Menyanthes trifoliata, Bog Bean

  5. how big if a pond can you treat and does it hurt the fish?

    • Nualgi is completely safe for fish, amphibians, and pets can safely drink from a pond treated with Nualgi too. It will actually enhance the color, health and activity of your fish. Any size pond, you dose 1ml per 1000 gallons. If your pond is larger than 500,000 gallons we suggest you contact us on as we can help with larger bodies of water too.

  6. My very small pond has algae growing on the side of the pond about 2 inches long. We scraped it off a month ago. The pond water was cleaned two weeks ago and you could see the bottom and fish. The weather has cooled here from over 100 during the summer down to the 80s. The pond was bad at the end of summer. Now we are in fall I am trying to clean it. Now I am feeding the fish weekly. The fish have been very healthy up to now. The pond is now soupy green and you can only see algae. What can I do?

  7. Can you guys donate this stuff by the tanker load to fight the problem occurring in Lake Okeechobee and the gulf?

    • We can certainly subsidize it but there is no one who is actually empowered to do something about it!


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