# [Wet Thumb Forum]-Duckweed as a light blocker/algae helper?



## Mr Fishies (Apr 9, 2006)

Can duckweed, that may have helped fight green water, contribute to growth of algae on and slowing the growth of submersed plants by blocking much of the light the same way green water would?

A few weeks back I was waiting out a green water algae bloom that was not going away. I decreased hours of light, moved my output spray bar down lower into the tank (about 2") to keep the duckweed from getting blown so it could multiply, allowed duckweed to cover the surface (about 90% coverage with filter flowing) and waited...and waited. Finally after watching it get so thick I could only see about 6-8 inches into the tank, I invested in a UV filter, plugged it in, did a water change and 4-5 days later, sparkling clean water.

It was like a whole new tank, it'd been so long since I saw it!

However, on almost all the plants in the tank, there formed a sparse furry coat of algae on them (shown in attached image).

Now, my spray bar output is bending submersed plants into strange contorted "bonsai snakes" with tons of root growth where they became closer to horizontal. However, if I move my spray bar back to the top, I will blow duckweed into all the plants. But since duckweed is growing so well, I can't decide if it's hurting the "keeper" plants by robbing them of nutrients and light, or helping them by keeping the algae from getting a lot of iron and light.

Tank is a 50 Gal with 96W 6700K CF
Last test results:

PH 8
GH 7
KH 8
NH 0 mg/l
NO2 0 mg/l
NO3 6 mg/l


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## dwalstad (Apr 14, 2006)

Duckweed probably helps. Its less competitive with plants than green water algae. Green water algae removes CO2 from the water and drives the pH up, making CO2 less available for plants.

In contrast, duckweed gets its CO2 from the air. It competes for light and other nutrients, but I suspect you have plenty of both.

It sounds like your filter is on steroids.







Can you not turn the holes towards the adjoining tank glass? That way the water jets bounce off the glass before traveling across the tank.

Some of those plants have no algae, and they look healthy. If this is a new tank, I would keep the duckweed and not worry too much.


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## Mr Fishies (Apr 9, 2006)

> Originally posted by Diana Walstad:
> ...
> It sounds like your filter is on steroids.
> 
> ...


Turn the holes towards the glass...Doh! Why didn't I see that?

It's certainly not an old tank, but that raises a good question, when does a tank stop being new? I've read the time for an El Natural tank to settle down is measured in months, and I've only had my tank up for 3-4 months. Is this time somewhat dependent on the "richness" of soil?

I used some of the same generic vegetable grade "no ferilizer added garden soil" in outdoor flower pots I just emptied this past weekend, and I must say it looked very dark and healthy still.

In looking for something with less organic material, has anyone ever tried using cactus potting soil to start a tank?

For now, I will continute to toss out a 3-4 cups (wet) of duckweed every week or two. Strange, the hardest part about my El Natural tank is having patience. The "hands off the aquarium" method of upkeep is hard to adjust to!


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## dwalstad (Apr 14, 2006)

> Originally posted by Fishies:
> Strange, the hardest part about my El Natural tank is having patience. The "hands off the aquarium" method of upkeep is hard to adjust to!


I've seen that in other hobbys. Many folks think that the more you work and spend on it, the better the outcome. That's not always true, especially when you're dealing with _Mother Nature_.









P.S. I think you're right that the time required for an "establish tank" depends a lot on the soil.


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## flagg (Nov 29, 2004)

Fishies: I just set up my el natural vase (see me other post) using cactus soil.... So far it's been a week and everything seems to be doing quite well.... I'll post updates on the other thread every week or two...

-ricardo


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## Mr Fishies (Apr 9, 2006)

> Originally posted by rick6805:
> Fishies: I just set up my el natural vase (see me other post) using cactus soil.... So far it's been a week and everything seems to be doing quite well.... I'll post updates on the other thread every week or two...
> 
> -ricardo


I'm going to keep a watch on that thread...I'm curious how it will go for you.

I am considering a small <= 5 Gal tank in the office at work, but I want to find a less rich soil to start with to try keep water changes to clear yellow/brown water from soil leaching humic acids etc.

I think as a result of rich soil, my water grows duckweed like crazy and there's algae on all the plants. As Diana pointed out, the plants are still healthy looking under the algae, but evertyhing that is not new growth has it. Even my snails!

There's dense adventitious root growth out of 75% of the sides of many L.Repens and H.Polysperma.

I've got 5 c.japonica who are giving select plants "crew cuts" and keeping the algae at about 3mm (1/8in) in the bottom half of the tank. Towards the top of a stand of B.Monnieri in the background of that shot the algae is about 1cm (3/8in).

(I wonder if ottos would venture up into the top branches more or less than the shrimp?)

It's not the aquascaped show tank some people strive for, but it has it's own character.


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## dwalstad (Apr 14, 2006)

How long has this tank been set up?

It sounds like its doing okay despite a temporary overload of nutrients. Be glad for that duckweed! In addition to duckweed, you could try adding a larger floating plant. Water sprite, frogbit, etc? I'm thinking that a larger floating plant might pull out nutrients even faster. More foliage above and more roots below....

Just a thought.


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## Mr Fishies (Apr 9, 2006)

The tank was filled and planted on July 24th. Tank is 189L (50G), 12L garden soil (3.17G), 14L gravel (3.70G). The garden soil was about 1-1.5 inches loose, I just added gravel over it. I knew it was fertile, rich soil, but it had no added ferts, no manure, and it came from a bag. (I was wary of taking soil from outside, 10% bugs, 90% ferts and other chemicals I don't know about).

I saw another post about C.Thalictroides, that tweaked my curiosity and I was going to look for it to be used as a stem plant, but it looks nice floating and emergent too. I've not been able to find it locally yet.

Frogbit, I had never really looked it up. I think it's scientific name is Limnobium spongia or Limnobium laevigatum? That looks like a really interesting plant. The L.laevigatum seems to be a flatter plant from the pics I found online. I have a glass cover for my tank so I can't easily grow tall out of water plants, but can I find frogbit?

I was thinking pulling the short V.spiralis from the middle of the back and adding a C.balansae, but water sprite would be a nice texture change from what's in there now. 

Since plants like duckweed and frogbit out-compete rooted plants by blocking light, at what point would I be blocking too much light? I already have about 100% coverage from duckweed (see pic).


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## flagg (Nov 29, 2004)

You know what you could do... add a small goldfish or two. That'll cut back on the duckweed for ya...

-ricardo


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## javalee (May 8, 2006)

Fishies,

Your tank is really beautiful, and the algae appears to be visible only under close inspection. In fact, the glow in the solid layer of duckweed roots is very beautiful.

I was just reading a scientific article about duckweed and related floating plants like frogbit. It was so fascinating. Apparently, these plants have a symbiotic relationship with bacteria within the plant which convert atmospheric Nitrogen into ammonia for the plant. The bacteria live within special bulbous structures in the plant. 

Point being that they may not be the best plant for removing nitrogenous compounds from the water given this information. This supports what Diana was saying about using watersprite. Have you ever tried hornwort and anacharis? I think that hornwort and anacharis both make compounds that inhibit algae and they can be used as floating or planted nutrient hogs.

Just wanted to share the neat info that was at least, new to me.


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## dwalstad (Apr 14, 2006)

I think Azolla (not duckweed or frogbit) is the Nitrogen-fixing floating plant you mean. The plant does this only when it doesn't have enough N in the water; if it can get N from the water, it doesn't need to fix it from the air. So adding Azolla to the tank shouldn't harm the system or increase ammonia, nitrates, etc. I have had Azolla; its a cute little plant. 

Rice farmers, who have to deal with nitrogen deficient soil/water, get free nitrogen fertilizer this way. They grow Azolla in their rice paddies to fertilize the rice. Pretty cool.

I hope that Robert (Hudson) offers an assortment of floating plants for hobbyists. If not, try pond suppliers.


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## javalee (May 8, 2006)

Ahhh, sorry for being a purveyor of misinformation! I thought that all Salvinia did this. I checked the article again and it said a common name of the Azolla was "duckweed fern" and that confused me.

I EDITED here. I think I keep screwing up so I'll keep my mouth shut/keep my keyboard locked and let people read the article posted below.









Again, sorry for the misunderstanding. I wouldn't want to steer anyone the wrong way!


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## Erin (Feb 18, 2005)

> That is another fascinating turn. I didn't realize it provided nitrogen to the plants around it. That could have useful implications for a natural tank as well. I can never get my nitrate above zero.


javalee,

Can you provide a link to that article? I am very interested and going to be really sad if it's true that the plant provided nitrogen to other plants around it because I just ditched a ton of it.









Regards,
Erin


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## javalee (May 8, 2006)

Hi Erin, 
About the plants providing nitrogen to those around them, I got that from Diana's post about rice farmers using it to fertilized their rice paddies. EDIT: I went back and found it in the article too. Apparently it's grown as a "green manure" and applied to the fields as fertilizer.

For what it's worth, I'll get that article for you because it was just neat to see how clever Mother Nature can be.

http://waynesword.palomar.edu/plnov98.htm

These biologist say they grow the azolla (not duckweed) in buckets of tapwater! I want some! It's even good in sandwiches they say







Sounds like someone was hanging out with Azolla a bit too much. Cool article though.


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## dwalstad (Apr 14, 2006)

Javalee,

Thanks for posting an informative and well-done website. I thought the Azolla story was really interesting when I was researching for my book. I wanted to weave it into the Natural tank story, but there just was no context to bring up the subject.

So I'm grateful that you introduced this interesting topic.

Azolla sandwiches, anyone?


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## jimjim (Jan 25, 2004)

Hi All: I'm new here but couldnt let this thread go by. I've been keeping planted tanks for about 40+ years now. 99% of them low tech..Two things(or Three) 1. Don't use goldfish for Duckweed control, they'll also eat the other plants. 2. Use common Australian Rainbow fish, they think Duckweed is candy. 3. I can get lots of Azolla here from the swamps(Used to collect native plants a lot) If you want some and Robert isnt selling it just PM me and I send you some for mailing cost(1-$2?)...Jim


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## GuppiesRfun (Apr 26, 2005)

Goldfish eat duckweed but reek havoc on other plants and I hear, shouldn't be mixed with tropicals. My duckweed in my 20ga. with swordtails and guppies grows fast, but I like it and have to throw out a bunch every once in awhile, but I give it to my goldfish in ponds out back.(No wasting duckweed! and is cheap food for goldfish.) I give it to my two fancy goldfish inside too, but I'm trying not to mix them with tropicals. I have hair algae on my little Java fern but that's because I gave some plants to a friend. Before, there was a jungle and it was great. Hope to throw more plants in. Even invested in a strange Amazon sword-type plant.-Paula


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## GuppiesRfun (Apr 26, 2005)

Just read JimJim's note about Australian Rainbows eating duckweed. I'm babysitting a big female Rainbow in my 20ga. with the duckweed. Didn't know they eat it. If the owner lets me keep her, don't I need more room for her? I have some swords and a few guppies with her and an algae eater.-Paula


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