There are three main types of problem algae found in ornamental ponds. First and most common is green water algae, or planktonic algae. This appears as a green tint to the water, often so thick that the water looks like pea soup with visibility of no more than an inch or two. The other types are string algae, which looks like clumps of fur growing on everything in the pond and blanket algae, which forms a free floating mat in the water. A thin coating of algae on all underwater surfaces is normal and good for the pond’s health.
Green water algae is the most common algae problem, but is not harmful to the fish. It is actually the best environment for newly hatched fry to grow in, but a pond that is too green to see fish in is no pleasure to own. There are several approaches to eliminating green water. Short term, chemical algaecides are effective, but they break down as they kill the algae, leaving nothing to prevent the algae from coming back a week later. Additionally, killing large quantities of green water algae at once will cause oxygen deprivation problems as all of the dead algae starts breaking down at once and stripping oxygen from the water. For this reason using a chemical algaecide in hot weather is particularly dangerous and a frequent cause of fish kills. A better approach is an ultraviolet light in the pond’s filtration system. As the algae passes in front of the light, the ultraviolet radiation kills it. The biggest problems with this method are the use of either too small a light for the pond volume or too large a pump for the size of light. In general, at least 1 watt of light is needed per 100 gallons of pond volume, with the pump then sized to achieve the recommended flow rate for that light. The third method of algae control is the use of a barley product. Many years ago, Scottish farmers noticed that when there was rotting barley in their farm ponds, algae did not grow. Small barley bales came to be used in ornamental ponds to control algae, but they are only effective when rotting, and take several weeks to rot enough to start working. Barley pellets and liquid extract were developed to give faster results. Barley products typically last for four to six weeks per application. The last method of algae control is to use plants to out compete the algae for nutrients and so starve the algae to death. Higher plants are more efficient at pulling nutrients from the water and leave the algae with no food to grow on. Water hyacinths are the single best plant for this. Most natural ponds will have a spring algae bloom as algae begins to grow earlier in the season than other plants. Once the other plants really start growing, the algae dies back.
We generally use a multipart approach to algae control – algaecide to kill what is there and plants with barley or ultraviolet light to keep it from coming back.
String and blanket algae will not be affected by an ultraviolet light as they won’t pass in front of the light to be irradiated. Powdered algaecides with oxygen releasing chemicals work very well to kill the algae, with plants and supplemental doses of a tablet algaecide to prevent its return. It is common to have string algae present in waterfalls and bogs, as this is where the highest concentrations of nutrients are. The bacteria in the falls or bog have just released the nutrients and the plants haven’t had a chance to remove them yet. Best solution? Plant plants to hide the algae in these locations.
Many on-line sources now tout ionizers or ion generators for algae control. While these work very well for all types of algae control, we will only recommend them for and sell them for water features that do not include fish. They work by releasing copper ions into the water, and while they do control algae, copper is very toxic to koi. The manufacturers of these claim that the amount of copper is too small to be harmful, but our tests here indicated otherwise and any major koi breeder or grower will use only where there are no fish or plants.