Algae is present in every pond, which isn’t a bad thing. In fact a moderate amount algae is beneficial for the overall balance of a pond. Algae becomes a problem when it grows out of control. Planctonic or free-floating can turn a pond murky and green, making difficult to see your fish. String algae or blanketweed is even more of a nuisance because it can smother pond plants, and even clog pumps and filters, possibly causing damage. New ponds and even established ponds during the spring often provide ideal conditions for excessive algae growth.

There are  2 main types of algae that are common in ponds:

1. Planktonic or Suspended Algae which causes green murky water. Sometimes resembling pea soup in extreme cases, making it difficult to see fish that are just a few inches below the surface of the water
2. Filamentous or String Algae, also known as blanketweed. This type of algae forms either dense mats or long threads around plants, rocks and other objects in the pond. It is also creates unsightly mats on waterfall and streams.

Algae is present in every pond, it occurs naturally and is part of a healthy ecosystem. Only when the pond becomes unbalanced, does it become a problem.    

• Dissolved Nutrients – Fish waste, decaying leaves, uneaten food (ammonia, nitrates, phosphates), minerals from tap water etc. provide food for excessive algae growth.
• Sunlight – inadequate plant coverage at the pond’s surface allows too much light into the pond enables algae to flourish.
• Warm Water – often caused by shallow ponds, inadequate plant cover, and poor circulation.

1. Do not drain the pond and completely change the water. Although the pond will look good for a few days or even a couple weeks, this will only upset any balance in the pond, and add a fresh supply of dissolved minerals to feed the algae and cause the process to start  over again. Plus, this is usually very stressful and possible dangerous for your fish.
2. Add Plants. Try to achieve a natural biological balance in the pond. Adding the right combination of plant material (oxygenating plants, water lilies & floating plants) will result in clear water without the need for chemicals which often only temporarily clear the water.
   • Cover 50-60% of the pond’s surface with plants. The best are floating plants like water hyacinth, water lettuce, duckweed or azolla because they multiply very quickly. Water lilies or lotus are also very good because they provide shade. also submerged plants such as hornwort work great as well. This should provide enough shade to keep the water cool as well as block enough sunlight to keep algae growth to a minimum.
   • Use one bunch of oxygenating plants for every 5-6 square feet of surface area. These plants compete with algae by absorbing excess dissolved nutrients and also provide O2. Plants such as hornwort, water thyme and pond weed are ideal.
3. Keep fish numbers at a reasonable level. A pond stocked heavily with fish is more likely to have significant algae growth. Waste from fish is converted in to nitrates, which contribute greatly to the growth of algae. If you have more than a few fish in the pond, a biological filter and/or ‘bog filter’ may be necessary to achieve satisfactory results.
4. Keep sludge and debris to a minimum. Periodically use a net or pond vacuum to clean the bottom of the pond. As sludge, dead leaves and algae break down in the pond, they promote algae growth. MuckOff contains natural bacteria and enzymes that digest sludge and help to reduce nutrients that feed algae.

Barley Straw Pellets or Barley Extract can be used to naturally control algae in the pond. Add it early in the spring because it takes a few weeks to begin to work. Sometimes string algae can get out of control very early in the spring or suddenly during very hot spells of weather.  Sometimes you want to get a handle on it quickly so when all else fails, Pond Balance, a product from Interpet, works wonderfully. It is not an algaecide, so it won’t harm any other plants (or even free floating algae for that matter), it works specifically on string algae. Add it in the spring and periodically through the summer. A few weeks after the initial treatments, the string algae will fall right off the rocks, liner or whatever it is growing on. Clarity Max Plus is another product that works in a similar manner in just a few treatments. ProFix a concentrated liquid algaecide, has become one of the most popular algae treatment because it works on both string algae and green water.

Suspended algae (pea soup green water) can sometimes also grow out of control no matter what you do.  First see our article How To Clear Green Water page for more information about controlling suspended algae, If you don’t want to use chemical and you have tried everything else, then consider installing an Ultraviolet Clarifier.

For Your Waterfall:

Sometimes it’s just your waterfall that grows a lot of algae, and the rest of the pond looks good. In this case, you don’t need to treat the whole pond, instead there are products that you can use as a spot treatment that kill algae on contact.  

• D-Solv Oxy Pond Cleaner is a white oxygen based powder that kills algae on contact in streams and waterfalls . Simply turn off your pump, and and remove any puddles or standing water from the stream. If there are any thick gobs of algae, it is best to remove them by hand first. Generously apply the D-Solv over any algae so that it is completely covered. The D-Solv goes to work immediately, oxidizing the algae. After about 15-20 minutes, when the algae is dead, turn on your pump.  I recommend placing a fine mesh net at the bottom of the waterfall or mouth of the stream to catch any dead algae and prevent it from going into the pond. 
• Rock & Fountain Cleaner works very much like D-Solv, except it is a liquid. This comes in handy if you have a high waterfall with a lot of vertical surfaces. It can be hard to get the D-Solv powder to stick to the vertical surface of a stone, but not the Rock & Fountain Cleaner. It come in a spray bottle for easy application to waterfalls, and other water features like basalt columns, fountains etc. Just spray it on the algae, wait 15-20 minutes and that’s it. You may need to reapply it if there is a thick mat of algae.