An aerial shot of two white koi fish in a pond with clear water and green and purple lily pads. There is a small fuschia flower floating on the surface.

What makes a pond green?

The more common algae that create the "pea soup" appearance in ponds is a single cell organism called planktonic or free-floating algae. It feeds on nutrients like nitrates and phosphorous and uses sunlight for photosynthesis. A bloom is when the algae quickly multiply and "take over" a pond. During an algae bloom, the warm water (which already holds less dissolved oxygen than colder water) can become less and less oxygenated and eventually hurt pondlife

Anaerobic vs. Aerobic Bacteria

Aerobic bacteria (oxygen-needing) quickly break down organic material into harmless compounds like carbon dioxide. Anaerobic bacteria (don't need oxygen) work much slower and break down the organic matter into nutrients that fuel algae and harmful gases like hydrogen sulfide, making the pond smell like rotten eggs. As the algae deplete the pond's oxygen levels, the aerobic bacteria die off, leaving the anaerobic bacteria to decompose the debris and sludge in the pond. Their byproducts create more nutrients for the algae, further fueling the bloom.

What Causes Algae?

Algae need direct sunlight and excess nutrients to flourish, which is one reason blooms tend to occur during the summertime. Algae love excess nutrients like phosphorus and nitrogen. These nutrients often are common in runoff, from fertilizers or organic material like leaves, lawn clippings, and dead plant parts. Overcrowding or overfeeding fish can also lead to nutrient overload in ponds. A still pond, one without fountains, waterfalls, or another source of aeration, allows algae to float on the surface and receive all of the sunlight it needs. When the water is moving, the algae aren't able to get as much direct sunlight.

Tips for Preventing Algae


Aeration is critical to help keep the water oxygenated year-round.  It will also keep the good anaerobic bacteria active during the summertime to break down the nutrients and debris. You can use a fountain, waterfall, or an aerator to keep the surface water constantly moving. 


There are three types of filtration systems: mechanical, biological, and UV clarifiers.  Mechanical filtration removes smaller debris by pushing the water through a filtration pad.  Biological filters utilize good bacteria to help break down nutrients and debris.  UV clarifiers help to break down the algae at a microscopic level using ultraviolet (UV) light. 

Aquatic plants

Many pond plants are dual-purpose decorations. Floating plants like lilies and water lettuce create shade and prevent algae from getting the sunlight it needs. Marginals like iris and pickerelweed help soak up the extra nutrients in the water.

Minimize debris

Pond nets can help leaves from entering your pond. You can also manually scoop out leaves and other debris daily.

Water quality tests

Checking water quality will help you detect any issues before they get out of control. You can check for excess nutrients like phosphates or nitrates and see how your oxygen levels are. You need to find the source of the excess nutrients so the algae don't keep coming back after treatments.

Add shade

Too much direct sunlight can heat the water, causing the water temperature to rise and the oxygen levels to drop. Also, algae need direct sunlight to photosynthesize. You can create more by adding the right types of plants, floating plants on the surface, or taller perimeter plants to block the sunlight. Artificial plants like the Floating Lilies or adding a treatment like Pond Tint also help to create shade. Read more about adding shade.

Water treatments

Preventative water treatments like beneficial bacteria, sludge remover, and pond tint can help prevent algae from taking over your pond.

Treating Algae


If the algae are very aggressive and have already taken over the pond or fountain, an algaecide may be a necessary step. Algaecides commonly use hydrogen peroxide and peroxyacetic acid to resolve an algae bloom. Read the instructions carefully and increase oxygenation during use to help prevent any fish deaths.

Complete Water Change

Algae can become very problematic, especially when reopening your pond in the spring and during the summertime. When algae bloom, they can quickly take over the pond, seemingly overnight. If you have tried using the smartpond® Algae Treatments, but the algae won't go away and are starting to affect your pond life, a complete water change may be necessary.