The Fickle Fishbowl ~ Duckweed

The Fickle Fishbowl ~ Duckweed

Today's Fickle Fishbowl column will be dealing with duckweed. Whether you call it by its scientific name (Lemnaceae minor), or by some of its more common names (duckweed, water lentils), or "You stupid piece of crap! Get the heck outta my tank and stop sticking to my fingers!" most aquarist who keep live plants have had some dealings with it. Why? Because, much like unwanted snails, duckweed is another hitchhiker that that we have to contend with.

Wanna see a mug shot? Taa Daa....

Pic from Plantedtank.net (pic shows actual size)

Awhh, it looks so cute and harmless, doesn't it? Don't be fooled! This stuff multiplies rapidly (even when it appears to be dead) and sticks to everything! Once you get a little piece of duckweed in your tank, it's a neverending battle to get rid of it. Worse, since it's a sticky little hitchhiker, you may be inadvertently transferring it to other tanks as well.

Why it's evil

Well, ok... I guess it's not truly "evil" but any time I'm doing water changes or filter maintenance, that's the word that pops into my head. Evil, evil, evil! I so much as look at my tanks containing this stuff and (I swear!) it jumps out of the tank and sticks on my hands, my clothes, and even in my hair. Eh, ok ~ I'm exaggerating a bit, but it seems that way! It does stick to your hands, your equipment and can get into and clog your filters and pipes. Don't believe me? Check this out:

pic from http://aqualandpetsplus.com/Plant,%20Duckweed.htm

Not so bad, you say? Just wipe it off, you say? Yeah well... see that tiny dot on the middle finger? That tiny little speck is all it will take to turn your beautifully planted tank into this:

Ew! Not so pretty now, is it? See it sticking to the bare glass? That stuff will dry out and look dead, only to be accidentally knocked off into the tank and WHAM! Duckweed soup all over again. Think I'm exaggerating this time? Some people have reported that duckweed can cover 2x its previous surface area in as little as 3 to 4 days! That's right ~ I said DAYS!

Consider that most people who are victims of duckweed hitchhikers are planted tank keepers. Duckweed creates yet another problem. Since it is a surface-dwelling plant, a thick mat of duckweed will quickly block all light to the rest of our plants ~ effectively making those expensive plant-friendly lighting upgrades useless. Can you say, "GRRRRR!"?

But it's a plant ~ good for the tank, right? Not necessarily. While it does help with water quality ~ absorbing ammonia and nitrate from the water~ it's not the best for maintaining a proper level of oxygen and carbon dioxide in the tank. Why? Well, I'll tell you. First, since this is a surface plant, most of the oxygen released by duckweed is released into the air. That's right ~ THE AIR! How is that beneficial to your fish? Second, if left unchecked, duckweed can quickly cover the entire surface of your tank. This limits the water's natural ability to outgas carbon dioxide. Is that important? You bet it is. Contrary to what most people believe, it is not the lack of oxygen that kills fish, but the concentration of carbon dioxide. If we don't have a large enough surface-to-air area in our tanks, we can kill our fish. Water circulation equipment (airstones for example) are beneficial NOT because they add oxygen to the water, but because they aggitate the surface of the water ~ allowing carbon dioxide to escape the water column. Hmmm, not looking good for duckweed.

Let's Be Fair

In the right situations, duckweed CAN be an asset to a fishkeeper. It really depends on what you need in your aquariums. If you keep fish that require super-high veggie content in their food, duckweed can help to balance out their diet ~ if they'll eat it. Duckweed has been reported to have laxative properties, so fish known for swim bladder disease or certain types of bloat could benefit from duckweed in their diet. Goldfish and koi are known to love this stuff! A few of my cichlids will nibble on it a bit, but because of the risk of clogging my filter (a HUGE no-no when keeping messy fish) I only add a few pieces at a time and watch to be sure it's eaten.

Use with caution! The preferred method of including duckweed in your fish's diet is to keep the plant in a separate tank, harvest and freeze it, and either mix it into your homemade fish food or feed it in small quanities as you would any other frozen food.

Another possible benefit of duckweed is that it provides cover for fry and shades the tank to help some fish feel more secure. It also provides a place for infusoria to thrive ~ first foods for many small fry.

So, duckweed can be food and shelter for certain fish, but is duckweed the best plant to provide this? That's a question that every aquarist has to answer for themselves ~ it depends. ;) Many plants can provide cover, if grown properly. Any fine-leaved plant (many mosses, such as java moss) can provide a place for infusoria to shelter. And many high-quality commercial and homemade foods do not contain duckweed. That said, duckweed is a plant we can definitely do without!

Well? What's your take on duckweed? Have a story to share? Let us know!