String Algae in Your Pond

Are you dealing with a String algae invasion? Here are some suggestions to help keep the algae under control.


What is String Algae?

String algae is a filamentous species that attach to plants, hangs from rocks in waterfalls, or hangs on the surface of the water. The long strands tangle together and form thick mats that can double their weight within 24 hours!


This mess can attach rocks and is not to be confused with the good algae that grow on the side of the pond. String algae are ugly, but not a major danger to your fish’s health.

A large amount string algae will reduce oxygen content, but doesn’t mean your water is bad. It is a sign of beneficial water qualities. Somes algae are acceptable since it contains a beneficial variety of carpet algae which is home to important micro-organisms that fully promote a healthy water column


How To Remove String Algae from Your Pond

To remove and reduce string algae you must scoop it out first and then get to the bottom of the issue in your water chemistry if it comes back.

Your waterfall will be the most difficult place to remove algae in your pond. Since the water rushes by so quickly it is difficult for water treatments to do their job effectively. To completely rid your waterfall of string algae it may take up to 2 seasons, so be patient and make sure you are not adding too many nutrients that encourage algae growth!


Steps for String Algae Removal & Reduction

  1. Physically Remove String Algae – This is your best method for removing string algae. Though it may seem dirty, it is essential to do so you can reduce the amount of decay. Pull the biggest bits of string algae near the base. Physical removal is the fastest way to get algae out of your pond. The hand approach is easiest. Wearing gloves is not required but may keep you cleaner. Use a long-handled brush to pull out the algae at depth.


  1. Treat Water and Kill Off Remaining Algae – Some people suggest using a pond algaecide to kill off the remaining algae but we do not recommend chemicals. You’ve worked hard to make a natural oasis. In the springtime, the good bacteria levels can be low. We recommend adding bacteria/enzymes to assist the natural process. Normal green hair-like or carpet/blanket algae which grow on pond walls and some rocks are best left untouched and completely acceptable.


  1. Add Extra Plants and Remove Decay –  Place quick growing and reproducing plants in your pond to increase oxygen content. Make sure you take out the decaying plants first, as they will not help your fight against algae. Choose plants that will grow larger, consume a lot of nutrients, and will not require a lot of upkeep. We recommend Water Lettuce, Irises, and Cattails for the spring and summer. Make sure you remove some of your extra plants from time to time to let new growth occur in your pond. You can solve many problems as a pond owner by placing plants to out-compete algae and suspend algae for excess nutrients. Just be careful to not add in any plants that already have string algae attached!


  1. Find the Cause of the Algae Growth – Look for potential causes of string algae by testing your water quality. If algae are growing at a problematic level than it is time to look beyond the algae and mat and deeper into the pond chemistry. High pH and Phosphorous levels are the leading cause of string algae. Examples of what can cause high pH are the clearing of algae blooms, excessive plant growth, overstocking of fish, and the introduction of foreign materials (untreated concrete, rocks containing limestone or calcium/granite). The most common cause of high phosphorous is from fertilizers that have leaked into the pond water. Iron is also a major contributor as well as grass clippings that find their way into the pond after mowing the lawn. Scoop green grass blades out immediately.


  1. Feed Koi & Fish Less to Reduce Excess Nutrients – One of the most common errors by fish pond owners is to overfeed their fish, thus adding excess nutrient to the water. If there is any food left in the pond uneaten, you’ve fed your fish too much.  By feeding less you also increase the fishes’ appetite for other substances in the pond. Like algae! Try feeding your fish less this summer and see how fast they will cut through a string algae mess. A few corbicula clams in the pond is a way to keep excess nutrients cleaned up and in check. Each clam filters up to a half a gallons of water per hour.


Should I Use Barley Straw and How Much?

Barley straw is a great organic product to use, but it doesn’t exactly kill existing algae. Barley creates conditions that prevent the new growth of algae. Put it in your pond early as it can potentially create a temporary algae bloom later in the season. Be careful, overdosing the pond with barley straw may cause fish kills, due to the straw de-oxygenating the water as it decays. Also, make sure you know where the barley comes from. Some farmers spray the barley fields to kill bugs.  These can still be on the barley straw and kill your fish.

2 lbs per 1000 gal of water are the recommended amount to use. In still and small pond waters the dosage of straw should be 2 oz per square yard of water surface area. When it is applied to cold water (less than 50°F), it may take six to eight weeks for the straw to begin producing the active chemicals that reduce algae. In warmer water above 70F, it becomes effective in as little as two weeks. In any case, barley straw remains effective for approximately six months after it is applied. If the straw starts to smell, it should be removed and replaced.


A Few Other Tips to Control Algae in Your Pond


1. Remove Leftover Decomposing Algae

Breaking down some of that excess organic matter is critical to controlling algae long term. Many ponds have from 3 to 24 inches of organic matter resting on the bottom. This organic matter releases excess nutrients as it decomposes and more so if the pond has a shortage of oxygen in its deeper parts. A more radical solution to removing the excessive organic matter is dredging or even draining the pond, cleaning out the bottom and starting over. This can definitely work but is very expensive. Also, once you clean out the pond, the problems can start all over again unless you take a proactive approach to managing excessive organic matter.

2. Add Extra Aeration

Adding aeration and circulation in your pond is the most important thing you can do to help prevent algae long term. Aeration increases the level of dissolved oxygen in the bottom part of the pond which increases the number of aerobic bacteria. These bacteria, in turn, begin to feed on the excess organic matter and reduce the number of nutrients released.

3. Add Beneficial Bacteria

Beneficial bacteria work at the decomposing excess organic matter, sticks, leaves, decayed fish and excess nutrients. They don’t have to be combined with aeration, but adding oxygen will significantly increase both their numbers and their level of activity at the bottom of your pond where you need them most.

4. Scoop and Remove Algae with a Rake/Eradicator

Another commonly used option for algae control is manually scraping with a rake or weed eradicator. These options are not for everyone and can require considerable time and effort to be effective.

5. Block the Sun

Is your pond getting direct Sunlight? One way to slow the growth of algae is to provide some shading for your pond.  This can give you time to let some of the other attempts to work.


Enjoy your Pond

Lowcountry Koi Club.