# Duckweed in the Natural Aquarium



## aquabillpers (Apr 13, 2006)

Hello,

Duckweed is a very nice plant in many situations. It provides shade where needed, overhead cover for shy fish, a source of food for fry and vegetarians, and it will consume a lot of excess nutrients.

I had a very nice 20 gallon natural tank, heavily planted. It was perhaps the best looking, healthiest tank that I have ever had. It needed no maintenance, except for the occasional removal of excess plants.

So I sort of ignored it.

One day I noted that the tank looked a bit dark and the plants were not doing very well. Investigation showed that the surface was covered almost completely with duckweed.

I removed much of the duckweed and the tank became brighter, but the plants continued to decline. I tested the water, and the nitrates were less than 1 PPM! That darn duckweed had consumed it all!

I made the appropriate adjustments, and the tank is doing a bit better, but I think I'm going to take it down and start anew.

Beware of the killer duckweed!  

Bill


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## Miss Fishy (May 13, 2006)

Are you sure it was the Duckweed that was causing the problem? I have Duckweed and/or other floating plants in all my tanks and ponds, and I usually only thin it out when the tank or pond surface gets completely covered. In addition to that, all my tanks and ponds except one have no measurable ammonia, nitrite or nitrate. Despite this, my plants grow very well. 

From Alex.


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## dwalstad (Apr 14, 2006)

aquabillpers said:


> Hello,
> I tested the water, and the nitrates were less than 1 PPM! That darn duckweed had consumed it all!
> 
> Bill


I doubt that your plants are nitrogen deficient or that duckweed has caused problems.

1 ppm nitrates means there's plenty of nitrogen in the tank. It means that the plants are getting enough ammonia nitrogen; the fact that there are measureable nitrates means that plants aren't nitrogen deficient. Only if your plants were yellow-colored (both old and young leaves) and you measured no nitrates, could you truly say the tank was nitrogen-deficient.

Usually, when plant growth tapers off in a tank after a year or so, it's because the soil organic matter has been decomposed; there's less CO2 for plants.

I'd keep the duckweed and start feeding your fish more.


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## aquabillpers (Apr 13, 2006)

dwalstad said:


> I doubt that your plants are nitrogen deficient or that duckweed has caused problems.
> 
> 1 ppm nitrates means there's plenty of nitrogen in the tank. It means that the plants are getting enough ammonia nitrogen; the fact that there are measureable nitrates means that plants aren't nitrogen deficient. Only if your plants were yellow-colored (both old and young leaves) and you measured no nitrates, could you truly say the tank was nitrogen-deficient.
> 
> ...


Thanks for your response.

The duckweed was so heavy that it blocked out a lot of the light that the plants needed.

The plants were not yellow, but most new growth had stopped, of the S. sublulata and the E. tennelus as well as the hygrophila.

The tank had a fair amount of mulm (from fish waste?) on top of the gravel, which I removed.

The fish looked well fed. If a nitrate level of 1 PPM or less is acceptable, why would feeding more accomplish anything?

Also, I had assumed that in a natural tank the CO2 level would be be at approximately the atrmospheric kevel, around 3 PPM. Are you saying that it would be higher in the first year of a soil-based tank?

Thanks.

Bill


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## dwalstad (Apr 14, 2006)

aquabillpers said:


> Thanks for your response.
> 
> The duckweed was so heavy that it blocked out a lot of the light that the plants needed.
> 
> ...


Hello Bill,
I've always recommended pruning and thinning out plants so that new growth can continue. That would include thinning out duckweed.

1 ppm nitrates is probably enough nitrogen, so you're right. There's no need to add more food. However, if symptoms indicate the plants need more nitrogen, I would suggest adding it as fishfood rather than as nitrate chemicals.

The 0.03-5% level in air does not translate into 3 ppm CO2 level in water. Clean water in equilibrium with air contains 0.5 ppm CO2 (0.5 mg/l). However, plant photosynthesis can quickly draw down even this small amount, so that by mid-morning the water may contain no CO2 (read 'Carbon's Scarcity in Natural Rreshwaters' on page 94 in my book).

In a soil-based tank, CO2 will be high during first year or so, because the decomposition of readily available organic matter in the soil will release CO2 into the water. As time goes on, this organic matter decomposition slows. Then the fishfood input is what helps provide fresh organic carbon for CO2 production.

In a natural tank, the plants depend on the CO2 release from fish respiration _and_ the bacterial decomposition of soil organic matter, debris, and dissolved organic matter. Unless the mulm becomes unsightly, I'd leave it in the tank.

Does this make sense?


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## aquabillpers (Apr 13, 2006)

Hi, Diana,

Everything that you say makes sense! That's why you have such a following. That, and your advice works, too.  

I've been trying for some time to reconcile your statement that the "equilibrium" CO2 level is 0.5 ppm. Just about everything I've been able to find says that dissolved CO2 in an aquarium ranges between 2 PPM and 4 PPM, at "equilibrium." My own experience in measuring CO2 / KH / PH relationships seems to support that. What am I missing?

BTW, I have a natural tank that has been set up for two years and is still growing strongly; a bit too strongly, actually. The natural processes in the tank must be supplying the nutrients that have been depleted from the soil.

Bill


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## dwalstad (Apr 14, 2006)

Thanks for your appreciative comments.

I stand by my statement that water in equilibrium with air is 0.5 mg/l. My Limnology reference book by R.G. Wetzel states (p. 202) that "the amount of CO2 dissolved in water from atmospheric concentrations is about 1.1 mg/l at 0 degrees C, 0.6 mg/l at 15C, and 0.4 mg/l at 30C."

This reference measurement was probably done using distilled water.

A beaker of distilled water is a lot different from aquarium water. In aquariums you have fish producing CO2. You have bacteria decomposing organic matter and producing CO2. And you have plants and algae taking up CO2 as they photosynthesize. The balance between these, and many many other water chemistry/substrate chemistry factors, will differ greatly between individual aquariums.

One study of natural waters containing aquatic plants showed CO2 levels ranging from 0-14 mg/l-- depending on the sample site and the time of day. This range probably applies to most planted aquariums.

A 2-4 mg/l level is a reasonable range for a single aquarium--- at a specified time of day. If you are measuring this much CO2, it could certainly explain why you're seeing good plant growth in your aquarium.

However, I would not generalize this value to _other_ aquariums (I haven't been able to measure any CO2 in my aquariums at late morning).

The CO2 input from air is generally considered by scientists to be a very poor carbon source for submerged aquatic plants.


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