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Duck Weed - Does it have a role


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I have duck weed in a few of my tanks that multiplies quickly. What do people see as the advantages. Is it a good agent for removing nitrates. Any problems except for having to remove the excess.

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I've found that my tanks with duck weed have no algea problems (ie. no green water, hair algea or spot algea) They do have some algea that grows on the glass around the filter, but other than that nothing. My tanks that don't have it have had the odd out break of the above mentioned algea. I also have duckweed in all my fry tanks- proberly covering about half the water- as it is good for fry to find mocriscopic food under.

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I agree with cosmiccreepers.....I have duckweed (Lemna minor) covering the surface of most of my fry tanks. I find that it helps to curtail the nitrate levels, provides food, dampens strong lighting and helps slow down the growth of algae (through aggressive competition for light and nutients). When profuse growth occurs, the excess is easily skimmed off and used as a supplement to the diet of many of my mbunas.


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hi dave

i have tried duckweed in the past, hoping to gain some benefit. apart from looking pretty for a week or two, no go. it just goes all brown and slimy. i don't know what i do wrong, no luck in tanks. can grow it outdoors. so now i'm trying my luck with ricca. cheers colfish

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I've tried the duckweed inside and I have colfish's problem with it, it doesn't survive, I believe in my case this is linked to light levels as I have fish lighting and not plant lighting on my tanks (OK for the java fern, java moss and anubias). Outside it grows everywhere watery, and it grows fast.

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Where do I get some???????!!!!!!

Just be aware that once you have some, it is very difficult to get rid of. You should also not flush them, as they are a weed.

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Some info I found.

Duckweed: Not just for ducks by Suzanne Kollar, PFRA, Beausejour and Darrell R. Corkal, PFRA, Saskatoon

A much maligned plant, duckweed has often been viewed as a nuisance, commonly mistaken for algae and associated with water quality problems in ponds, dugouts and stagnant water bodies.

But the facts are that duckweed will remove plant nutrients from water, block sunlight and out compete algae. It can even reduce evaporation loss from a dugout.

While duckweed is an indicator that excessive nutrients exist in the water, it doesn't contribute to water quality problems. In fact, this macrophyte plant improves water quality by removing phosphorus and nitrogen from the water and by naturally filtering unwanted matter in the water.

With optimum conditions--food, sunlight and shelter from wind--duckweed can grow exponentially by consuming phosphorus and out compete algae (phytoplankton), which is lower on the food chain.

Duckweed growth, instead of algae, is very desirable since algae pose more problems for water use. Some algae can produce liver or nervous system toxins--for example cyanobacteria or blue-green algae; other green or brown algae species are often so small that they will pass through or plug water treatment filters; and, almost all algae will cause taste and odor problems in water.

Duckweed is an oval shaped plant that floats on the surface of water. It is the smallest flowering plant. When mature, the smallest species is two mm or less in diameter, and the largest species is about 20 mm in diameter, roughly the size of a fingernail or thumbnail. Duckweed looks like tiny floating leaves on the water surface. There are four duckweed genuses-- Lemna, Spirodela, Wolffia, and Wolffiella--with over 40 species identified to date. The most common genus in Canada is Lemna.

Often spread by aquatic birds and floods, duckweed grows in clusters and can grow rapidly with adequate food (phosphorus and nitrogen), sunlight and shelter from wind. Commonplace worldwide and quite hardy, it will even tolerate brackish water (up to 4,000 mg/L of total dissolved solids).

Often considered unsightly, duckweed blooms can cover an entire water body with a "green blanket" or "mat" containing millions of the small plants. It won't thrive on sites exposed to wind or where flowing water occurs.

When blanket of duckweed covers a dugout, it limits growing conditions for algae.

This means the plant food for algae will have been reduced as the duckweed blanket blocks sunlight to the water column, limiting photosynthesis and preventing algae growth.

When duckweed covers a dugout, it's desirable to check the oxygen levels in the water. A simple test indicates low oxygen levels:

While on a dock or boat, scoop some of the duckweed away and rapidly collect a water sample using a wide-brimmed container such as a jar.

Check for the presence of small animals (zooplankton) swimming in the water. A healthy water body will have many of these beneficial animals.

If the zooplankton have a reddish appearance (its hemoglobin), the oxygen levels are too low and supplemental aeration is required.

Managing your dugout

In fact, it's recommended that all dugouts (whether or not they have duckweed) be continuously aerated year-round using good diffusion systems such as air stones or specially-designed linear diffusers.

A carefully managed dugout should include culling or harvesting the dead and decaying plant matter each fall, including the duckweed plants.

Do this before water temperatures get too cold. This will remove unwanted nutrients from being recycled into the water body as plant matter decomposes.

If duckweed isn't harvested regularly, the decaying plant matter will turn the dugout water a brownish color.

Duckweed has been harvested by dragging floating timber booms across the water towards shore, and then removing the plants from the watershed to prevent them from decaying or re-entering the water.

With proper management techniques, harvested duckweed from wastewater lagoons may be fed to fish or livestock, including poultry.

Harvesting also ensures a continuous new supply of healthy, young duckweed plants that are more efficient in the uptake of nutrients than older plants. Over time, these measures will also reduce the growth of plants in the dugout.

If duckweed populations are excessive, nutrient inputs into the water body (or internal nutrient recycling within the water body) must also be excessive. To stop external nutrient inputs, it's necessary to limit runoff containing fertilizers, pesticides, animal waste and sediment.

To manage internal recycling, the water body should receive 24-hour diffused aeration year-round. Chemical intervention is sometimes used, but repeated treatments would be required since chemicals won't solve the problem of excessive nutrients in the water body.

Duckweed doesn't only have water quality control benefits for dugouts. Some studies suggest that a complete coverage of duckweed will reduce water loss from evaporation by as much as 33 per cent when compared to open water bodies.

A natural water purifier, duckweed has also been successfully used to treat sewage by bioaccumulation, removing as much as 99 per cent of the nutrients and total dissolved solids in wastewater.

Duckweed has also been used very successfully to sustain oxygen levels. Some duckweed species grow in stagnant, polluted waters, which also makes this plant ideal for water reclamation areas.

Acknowledgment: Much of the information in this article is based upon data presented in Duckweed Aquaculture--A New Aquatic Farming System for Developing Countries by P. Skillicorn, W. Spira, and W. Journey, funded by The World Bank, Emena Technical Department, Agriculture Division.

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