How to Get Rid of Duckweed in a Fish Tank

In September somebody in r/PlantedTank held up a dripping handful of duckweed and told every beginner on the sub never to get it. Third handful that size in a week. A few replies down, the same person admitted they keep some in on purpose, because it took their nitrates to nothing.

That's duckweed in one thread: the plant some keepers prize for what it eats, and everyone else spends their Sundays scooping out.

If your tank has grown a green lid and you want the light back, start with the free step. The gadget comes second, and it isn't for every tank.

Will duckweed go away on its own?

No. Left alone it doubles every few days, and pulling out most of it buys a week or two before the surface closes again. The filter is part of the problem. Leaves get sucked in and spat back out days later, still alive. A keeper on the Aquarium Co-Op forum puts the whole war in six words: miss one piece and it's back.

Some keepers don't want it gone. It's a nitrate sponge, and more than one reply in that thread keeps a handful going because it's worth the clean-up. Everything below is for the rest of us.

Start with a comb

The tool most of the sub reaches for is a plastic hair comb. Draw it across the surface and the leaves catch between the teeth and lift out. One reply to the beginner warning got rid of it entirely this way, and the whole method fits in three sentences.

If you're looking to get rid of it entirely, I've had success using a hair comb to lift it off the water surface. Do that everyday for about 3 weeks and you should be in good shape. Takes a while but it's simple and effective.
u/GClayton357, in r/PlantedTank

A cheap plastic comb, every day for a few weeks. The replies add the refinements. Snip the wide ends off so it reaches into the corners. A wide-toothed hair pick pulls out more per pass and knocks clean easily. Another finishes with a paper towel around the rim, where the last leaves cling. Someone who'd fought it a long while summed up their win as "comb + diligence wins the war".

Rinsing is the slow part. On a big tank the comb fills faster than you can clean it. That keeper's fix was a small water pump in a bucket, pouring over a mesh strainer. Comb, hold the comb in the flow, comb again, and nothing goes down the sink.

The comb isn't sacred. Replies in the same threads use dollar-store strainers, forks or the slotted spoon sold for fishing olives out of a jar. A keeper pushes an empty bottle in at an angle, so the surface water pours in with the leaves on it.

The fish net you already own is the one tool people complain about. It holds onto the leaves, they dry in the mesh, and every net in the house ends up with bits in it.

Digby draws a dark plastic comb across the tank's surface, a stripe of green leaves caught in its teeth and a clear lane of water opening behind it. A strainer of wet leaves rests on a raspberry bucket beside him, and he looks at ease. — The comb lifts the leaves out between its teeth. A pass a day, and the carpet goes before the specks do.

Does a surface skimmer get rid of duckweed?

The comb gets the carpet. What it doesn't get is the specks you can't see, and a few of those are enough to start again. For them, several keepers who've won for good put a small internal surface skimmer in the tank. It's a pump that hangs at the waterline, draws in the top skin of water and collects whatever was floating on it in a chamber.

One keeper says they've cleared duckweed entirely from several tanks with nothing but a skimmer. A day of cleaning by hand, then let it run. Asked which one, they said the cheapest on Amazon, and that they kept stirring the surface and wiping the glass while it worked.

Another keeper's version is to clean out most of it, then let the skimmer collect what's barely visible. A third names a cheap Aquaneat surface skimmer stuck to the front glass, facing inward, as long as you've no other floaters to lose.

The other brand named is the Aquatop surface skimmer. Its owner in the thread says it takes duckweed from a pest to a weekly chore: one big first clean, then once a week.

Their set-up comes with a catch, though. They took the safety top off and sank the intake until a vortex formed. In the same comment they warn that their shrimp want to ride it. We'd leave the top on. It's the one thing between the shrimp and the pump.

A keeper who vacuums duckweed out with a skimmer says a stock one needed its openings enlarged before the leaves would go through. Keep it running a few days after the last visible piece, they add. A planted-tank shop's own guide says the same: leave the skimmer on for a week or two after the clean-up, and empty it, because it holds only so much.

A skimmer is for the specks you can't see, after the comb has taken the carpet, and the owners who rate it ran it for weeks.

Who should skip the skimmer

Baby shrimp. Anything that pulls in surface water pulls them too. One shrimp keeper can't even use a strainer without tongs and a rinse to get the babies out first, and the Aquatop owner above had the same trouble.
Other floating plants you want to keep. A skimmer can't tell duckweed from frogbit. On the UK Aquatic Plant Society forum a keeper says that with salvinia in the tank you have no chance of losing one without the other. Frogbit and water lettuce can be lifted out and rinsed through several buckets before the leftover duckweed is spooned out, but one missed frond starts it all over.
A filter outlet in reach of the leaves. A keeper gave up on theirs because duckweed clogged the filter outlet while it ran.

Getting the last pieces

Clean the filter, then look every day. Duckweed rides through a filter alive, so check the media and the outlet for the odd leaf. The keeper who came back to the sub with an update says what finally worked was fingers: every little piece, then the filter, the glass and the other plants.

After that it's a daily habit. The oldest thread we read, from 2015, has the habit in four words: see one, remove it. A UV light helps here, because duckweed lights up under it and the hidden bits show.

Keep the surface moving as well. Keepers on the forums say surface agitation thinned theirs out or cleared it, and replies on Reddit recommend it too: spray bars angled at the surface, air-driven sponge filters or high-flow filters. Even a small fan across the top bunches the leaves up, which makes them easier to take out.

If you'd rather keep a patch

For keepers who want the nitrate sponge without the lid, the sub's answer is a ring. Take a length of airline tubing, melt the ends together into a circle and float it. The duckweed stays outside, the light gets in, and a comb pulls out whatever drifts in. One keeper runs it the other way round, with the duckweed corralled inside a ring cut from a deli-cup lid.

It isn't tidy, though. One keeper picks leaves out of the ring every day, and another says any bump to the tank gets them under it. A floating ring of airline tubing keeps a window open for as long as you keep clearing it.

The tank surface is clear, with green plants and small orange fish plainly visible. Digby leans an elbow on the sideboard with a red mug in his paw, looking at the fish with a pleased, relaxed smile.

Start tonight with the comb, before you order anything. A pass a day for a few weeks is what worked for the keepers who got it all out, and it shows you what you're left with. The skimmer comes second, for the specks, once the carpet is gone and nothing in the tank is small enough to ride it.

Two came up by name: the Aquaneat, which a keeper stuck to the front glass and left to run, and the Aquatop, down to a weekly job for its owner after one big first clean.