String algae

Asked August 27, 2013, 8:59 AM EDT

Outdoor pond..approx. 40x60 and 2 feet deep(with fish and turtles).....filled with string algae for first time in 40 years. Can we use hydrogen peroxide to correct this without harming animals? How much peroxide and should we rake out as much old algae as possible before treating? Any information regarding cause, cure, and prevention is greatly appreciated. Thank you so much!

Montgomery County Pennsylvania

1 Response

I am not sure if peroxide is the first step I would recommend as it may make a plastic pond liner brittle. Try the following remedy first from Bob Souvestre:

String algae (filamentous algae) growing on the sides of the pond is advantageous and beneficial to maintaining a healthy pond. The string algae that develops more as a free floating algae is problematic and requires a multiple-step approach to immediate and long term management. Accomplishing any one, or combination of these steps, will help manage algae growth. Step One: Physically remove as much of the algae as possible. If significant organic matter is present on the bottom of the pond, remove it as well since the breakdown of this organic matter will release nutrients into the water that benefit algae growth. If you typically fertilize the plants, especially water lilies, wait until you get the algae under control. Reduce, or stop feeding the fish until the algae is controlled. Step Two: Prevent any surface runoff from lawns and gardens to enter the pond. This will eliminate excessive nutrients from entering the pond and upsetting the balance of a healthy pond. Step Three: Optional – use a non-toxic water dye to reduce the amount of sunlight entering the water. This may negatively influence the bright coloration of fish – they may not be as colorful but it is harmless to fish and plants when used according to the label. This is a at best a short term solution and must be reapplied. Step Four: Aerate the water, preferably by using a pump connected to a pipe or nozzle. Step Five: Add rock to the pond to increase its overall surface area. Beneficial bacteria will colonize the rocks and use nutrients in the water thus starving the algae. Step Six: Optional – purchase a bacterial formulation labeled to ‘jump start’ the bacterial action in the pond. Step Seven: Optional – use either barley pellets or barley hay. Step Eight: Increase submersed and floating plants in the pond. They will offer both shade and utilize nutrients in the water thus limiting algae growth. Step Nine: Aquatic herbicides may be used but are not recommended for ornamental ponds since many are sold in large sizes and some can injure fish and plants.

Thanks for your question,