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Fish-Safe Algaecides for Pond Management

Algaecides to eliminate pond algae blooms come in a variety of forms. Both liquid and granular algaecides are effective for managing algae blooms in most bodies of water. 

Liquid algaecides are fast-acting and most effective for floating and planktonic algae that turn water green. Pea soup green pond water is a sign of looming pond management problems. Planktonic algae use up oxygen at night which can cause stress or even a fish kill if left untreated.

Liquid treatments should be used with a surfactant, like Plexmate, that helps the chemical algaecide penetrate and stick to the algae to work more quickly. The surfactant and liquid algaecide can be mixed with water in a tank sprayer for easy application. Liquid pond algae killers are a bright blue color because of the copper in the treatment. This makes it very easy to see where the treatment is sprayed to be sure that all of the target areas are treated.

Granular products like Cutrine Plus Granular control (filamentous) string algae for longer time periods. that typically begins to grow on the pond bottom before floating to the surface as green pond scum. The easy-to-apply granular treatments can be effective for as long as five weeks. Application is simple and can be completed by hand broadcasting or using a seed spreader mounted on a small boat.

A word of caution, effective treatment of filamentous algae will cause the pond to look worse before it looks better! The dead and dying plants float to the surface in green to brown mats. It can be helpful to remove string algae that is dead so that nutrients from the decay process don't contribute to another bloom.

All algaecides should be combined with a long-term pond management plan that includes treatment for excess nutrients to prevent large algae blooms. Effective pond algae control improves water quality, reduces biological oxygen demand, and results in a healthy pond. While some nutrients are good for pond productivity, out-of-balance nutrients are the main cause of blooms. Phosphorus and nitrogen are the nutrients that most often contribute to excess string algae in your pond.

To protect fish and other aquatic life algae control for ponds should always be accompanied by pond aeration. Dead and dying algae consume oxygen as they decompose. If left without effective aeration, the oxygen levels in the pond can reach dangerously low levels causing fish mortality.

The most effective pond aeration system to use after treatment is a bottom aerator that circulates water and increases dissolved oxygen. These very efficient pond circulators are easy to install and use small amounts of electricity.

Why are algae blooms bad?

When algae blooms become severe they can literally suffocate a water body. Excess algae impacts fish and other aquatic life by degrading habitat conditions. Algae blooms also limit recreational use of ponds and lakes.

Algae, like any other organism that uses photosynthesis produces oxygen during daylight hours. The problem is that algae uses oxygen 24/7 while it's alive and even after it dies.

Harmful algae blooms (HABS)

HABS are a growing water quality problem in water bodies around the world. From relatively small ponds to massive lakes such as Lake Erie, HABS can impact drinking water supplies as well as human and animal health.

The name Harmful Algal Bloom is actually a misnomer because the HABS are cyanobacteria. The visual appearance of the cyanobacteria in the water mimics the appearance of algae which explains why it is also known as blue-green algae.


There are many species of cyanobacteria. Based on the species, the visible cyanobacteria characteristics vary. Planktothrix forms a green pollen-like film covering the surface. Mycrosystis and Dolichospermum form a green paint-like slick on the water surface and near shorelines. As the surface films get older, they can appear to bubble and may take on different colors.

Other cyanobacteria species are found below the surface of the water which makes detection difficult. In order to confirm that a bloom is cyanobacteria, it must be tested. You cannot reach a conclusion on visual inspection alone.

Cyanobacteria Health Effects

Exposure to cyanotoxins released by Blue-green algae can have extremely serious health impacts for humans, domestic animals, wildlife, and aquatic life.

Algal Toxins are released during the death and rupture of cyanobacteria cells. Toxins including endotoxins, neurotoxins, and hepatoxins can impact human health. Exposure to cyanotoxins can occur from contact with water, ingestion of contaminated water, and breathing airborne toxins near affected waters.

Effects of exposure include skin irritation(endotoxin), abdominal pain(endotoxin), headache(endotoxin), vomiting(endotoxin), diarrhea(endotoxin), respiratory disease(neurotoxin), paralysis (neurotoxin), liver failure(hepatoxin)and in severe cases, death.