Adventures With Duckweed
By Dwight D. Moody, TFCB
TFCB May, 2014
Duckweed (Lemna minor) is a common aquarium plant. It is commonly found in the wild and also tends to be offered at a very reasonable price at fish club auctions, AquaBid.com and other places. It can also be collected from the wild, but one must be careful to not introduce fish enemies, such as dragonfly larvae. It is a great plant for beginners because it is easy to grow without any special substrates, CO2 injection, special lights, special fertilizers, etc. Many people have mixed feelings about it, so they tend to either love it for its benefits or hate it because it tends to propagate rapidly and is difficult to fully eradicate.
Duckweed is a very simple plant, it just sits on the surface with a few small leaves with a few, fine roots that hang down. Its primary benefits come when there is a good sized group of them. Just like any plant, duckweed takes nutrients produced by the fish out of the aquarium system (nitrates, phosphates, potassium, etc., generated by fish poop, uneaten food, etc.). Unlike most plants, it is a plant that a novice beginner can put in their aquarium and have a thriving crop of duckweed in no time, with no need for fertilizer, CO2 injection, special substrates, etc. That thriving mass of duckweed will do wonders to take all kinds of things that degrade water quality out of the system. All it needs it a little light.
Duckweed is a good plant for various Gouramis to build their bubble nests underneath. The bubble nests stay together much better with duckweed plants around them that provide support and keep the bubble nest from breaking up. Duckweed can also serve as a fairly good cover for fry and will tend to have a lot of small micro-organisms on the roots that fry can nibble on. If you are going to use it for Gourami breeding, try to have another plant in the tank, such as a big ball of Java Moss (another great beginner plant) or even a plastic plant designed for fry cover, because once the fry get to a certain point, those fine little roots will not hide them very well. Some Gouramis will not bother their fry, others will eat them, so it may be best to remove the adults unless you want to see whether or not they bother their fry. In my experience, adults that have enough high-protein foods do not bother the kids. Suggested foods include grindal worms, brine shrimp, bloodworms and daphnia.
Duckweed may also be a good food for fish that love to eat plants. Goldfish, in particular, LOVE duckweed, or so I am told by Dave Isham of the Tropical Fish Club of Burlington, who also says it can be used as a human food (although I have yet to see any of the vegetarians I work with eat duckweed burgers for lunch). Some fish cannot be kept in aquariums with regular plants, because they love to munch on live plants. For these fish, keep them with plastic plants which look good but cannot be eaten. Unfortunately, then the problem is how to give them the plant nutrition they need without paying $10 a plant, which is where a culture of duckweed can be a great help. Duckweed harvested from another aquarium purifies the water of the aquarium it is grown in as well as can be a great food for the fish in the other aquarium.
Duckweed’s negative aspects basically are outgrowths of its positive aspects. Once it is in an aquarium, it tends to totally cover the surface of the aquarium, which may limit the light available to other plants which are lower in the water column. Because it buds off daughter plants, just a few plants can rapidly repopulate the aquarium surface, which can make it really hard to remove from an aquarium once it gets established. Sort of like dandelions in your lawn, remember that the second part of duckweed is “weed”. I do, however, have some techniques that you may find helpful if you decide that your duckweed has overstayed its welcome.
To remove duckweed, you need to remember that it has a major liability: DUCKWEED FLOATS. The first thing I do when I want to thin it out is to add some water to the aquarium, which tends to cause the duckweed to float to the top, with the other plants, even floating plants, somewhat lower in the water column. I then use a large, fine mesh net to skim as much as duckweed off the surface as I can, putting it into a container as I accumulate duckweed balls of skimmed duckweed. Areas that are difficult to access with the net can be skimmed with a turkey baster, just use it to vacuum up loose duckweed in the corners and around filters, heaters, etc., then squirt it into a fine mesh net to filter out the duckweed. Keep in mind that you are NOT going to get it all the first time. So plan to do it again in another few days, that way you get more of the duckweed you missed and any tiny daughter plants will have grown enough to be captured and filtered out. This may take a couple of weeks to get it all. Keep in mind that once you get it ALL eradicated, all it takes is a few plants to get back in and you are back to the beginning.
When you see that bag of duckweed up at the fish club auction for only $1.00, think about both the benefits and the drawbacks of your potential purchase. It can give you $100 worth of aquarium filtration and gourami breeding plants, as well as serving as a live food culture for plant eating fish. On the other hand, if not in a planned situation, it can hard to eradicate, so you need a plan on how to keep it out of aquariums where it may not be beneficial, which can be as simple as dedicated nets to prevent introducing duckweed into aquariums you do not want it to get established or rinsing nets thoroughly before using them in a different aquarium. Duckweed is a great beginner plant and even those of us with more experience may find value in it, keeping in mind the issues that it has. Once you are comfortable with duckweed, you may want to graduate to other floater plants such as Frogbit (Limnobium spp.), which is like duckweed on steroids, a LOT bigger, a lot more complicated roots that are dense enough to hide livebearer fry but also more like a dollar a plant.