Duckweed (Lemnaceae), also known as water lens and bayroot, is a tiny floating plant, that holds the distinction of being one of the smallest flowering plants in existence.
It can be found in nearly every type of aquatic environment, but only truly thrives and proliferates in environments that have little in the way of current. In wetlands and ponds, where the water is almost completely still, duckweed can quickly overrun the entire surface of the pond. If it is allowed to completely cover the surface of the water, it can lead to oxygen deprivation, and may lead to large-scale fish die-offs.
Duckweed will thrive in nearly any aquarium, and in most cases will experience explosive growth thanks to the fertile environment that aquariums offer. Because of their impressive growth rate, they act as one of the best water purifiers that you can find for a fish tank and can be invaluable to newly established aquariums that are having water quality problems.
And if cleaning and purifying your aquarium’s water isn’t enough, they also have a well deserved reputation for reducing algae growth. Between the reduced light reaching the aquarium water, and the quick uptake of nutrients, any tank containing duckweed in anything approaching significant numbers will experience greatly reduced algae growth.
Of course everything comes with a downside, and once you have duckweed in a tank, it is almost impossible to ever completely remove it. It also has the annoying habit of clinging to everything that it touches, and anything that you remove from your aquarium, will end up looking like it’s covered with a forest of duckweed.
Duckweed doesn’t require any special care, and will thrive in everything from low light to high light, and soft water to hard water conditions. It’s actually more difficult to get the duckweed not to grow, than it is to just sit back and allow it to grow on it’s own.
If you are looking for heavy growth, then provide it with a high quality, full spectrum light, with trace minerals adding during the weekly water change. The one that I personally add after every water change is Seachem Flourish Trace Elements. Also, make sure that the surface of the aquarium is completely tranquil, as even a small amount of current will slow down the growth tremendously.
Duckweed mostly reproduces through asexual budding, but in rare cases it may also reproduce sexually. Its reproduction doesn’t require any intervention from the aquarist, and if the conditions are right, it will reproduce on its own.
Duckweed is compatible with most small fish, but will be hungrily devoured by herbivore fish and some cichlids. And that’s not necessarily a bad thing, since it is packed with nutrients and protein, and can even be used as a primary food source for some fish like goldfish and tilapia.
Of course if you want to cultivate duckweed, it should only be kept in tanks with smaller, non-herbivorous fish and even these may occasional eat small amounts. The fish that should be avoided are large cichlids, goldfish, ameca splendens and common plecos.