How to Control Duckweed in Your Aquarium

Duckweed is actually a great plant for aquariums. It grows fast, takes up nutrients from the water and competes with algae. Fish and shrimp find shade and cover underneath. But anyone who has had it in their tank knows the problem: after a few weeks the whole surface is green and it is hard to get rid of.

In this article I show you how to keep duckweed under control without banning it completely.

Why duckweed becomes a problem so quickly

Duckweed multiplies by splitting off new leaves from the mother plant. Under good conditions, with plenty of light and nutrients, the amount can double within a few days. On top of that, the filter current spreads it evenly across the whole tank.

Once the surface is completely covered, there are a few downsides:

• Less light for the plants below. Stem plants and carpeting plants start to struggle.
• Less gas exchange. A densely covered surface slows down the exchange between water and air.
• Clogged filter. Single leaves get sucked in and stick to the intake strainer or the filter media.
• Food doesn't get through. Flakes get caught between the leaves and the fish can't reach them easily.

1. Skim regularly

The easiest method is to remove part of the duckweed regularly. Ideally you do it during your weekly water change, so it becomes routine. A fine net works much better than your hand because it also catches the tiny leaves.

For exactly this purpose I designed a duckweed net. The shovel shape picks up the duckweed across a wide area, and the fine hole pattern lets the water drain instantly while the plants stay in. Usually one pass over the surface is enough.

You don't have to throw the removed duckweed away. Many fish love to eat it, and it is also fine in a garden pond or on the compost. Please never dump it into natural waters.

2. Use a floating ring

If you want to keep duckweed, but only in one spot, a floating ring is the cleanest solution. The ring sits on the water surface and the duckweed stays inside. The rest of the surface stays clear, the plants below get light and the filter stays clean.

There are two options:

• Free floating: The ring drifts with the current. It looks natural, but it moves around the tank.
• With suction cup: The ring is attached to the glass and always stays in the same spot. Handy if you want the duckweed in a specific corner or away from the filter.

The size you need depends on how much area the duckweed should have. For nano tanks 50 to 75 mm is enough. In most tanks from about 54 litres, 100 to 150 mm works well. For a really large floating plant zone, go for 175 to 225 mm. You can find all shapes and sizes in my floating rings collection.

A practical tip: the duckweed keeps multiplying inside the ring too. When it is full, just take some out. That is quick, because everything is in one place.

3. Keep an eye on nutrients

The more nutrients in the water, the faster duckweed grows. If it explodes, that is often a sign of too much nitrate or phosphate. It helps to fertilise a little less, not to overfeed and to change water regularly. That way it grows more slowly and you need to skim less often.

4. Adjust the current

If the filter outlet moves the surface a lot, the duckweed is constantly spread around and ends up in the intake faster. It can help to lower the outlet a bit or point it in a different direction. A foam prefilter on the intake also stops leaves from getting into the filter.

Can you get rid of duckweed completely?

Honestly, it is quite a lot of work. A single leaf you missed is enough for it to come back. If you really want it gone, you need to skim everything consistently for several weeks and also check the filter and the intake. Rinse new plants thoroughly before adding them, because that is often exactly how duckweed gets into the tank by accident.

For most people the better way is to keep it and simply hold it in one place. Then you get the benefits, nutrient uptake and shade, without it taking over the whole tank.

Quick summary

• Skim part of it once a week with a fine net.
• Keep the duckweed in one place with a floating ring.
• Don't overfertilise or overfeed.
• Protect the filter from leaves getting sucked in.

If you have questions about your tank, feel free to write to me via the contact page.