How to Get Rid of Duckweed in Fish Ponds 2019 (Fish Safe Methods)
What is Duckweed exactly?
Duckweed, or water lens, is a family of small flowing plants that float on the surface ponds and lakes, similar to water lilies. They’re more common in slow moving or stagnant bodies of water, but can also thrive in more aerated ponds if nutrient levels are high. Unlike algae, which compromise a group of photosynthesising micro-organisms, duckweed is an actual plant and can have similar benefits to ponds that come from regular aquatic plants. For this reason, so long as duckweed is not allowed to become invasive and take over the entire pond, small levels are usually left alone as they provide extra water filtration by making use of extra nutrients. Duckweed also provides a good snack for goldfish and koi; with goldfish in particular being very fond of the taste!
Even so, duckweed is notorious for its fast spreading capabilities under the right conditions, and can quickly become dangerous, especially in small ponds with fish. It’s always best to keep duckweed levels low in fish ponds by regularly removing any overgrowth before it can spread. If levels are allowed to get too high the duckweed will begin to reduce oxygen levels and contribute to an increase in waste, sludge, and harmful substance build-up.
If you find a sudden duckweed bloom it could be a sign of a growing water quality problem that requires further investigation. Often it’s not enough to just remove the duckweed, as it will just come back in future (often even faster) if there are lingering issues with water balance.
Where does Duckweed come from?
Often duckweed is carried to a pond by birds or other animals; starting as a small floating seed which is very difficult to identity until it grows. Once a single instance of duckweed has been introduced to a pond it can quickly spread under the right conditions, so the best time to remove duckweed is as soon as you see the first few sprouts appear. Duckweed can also be carried with other aquatic plants you purchase, usually hidden away among the foliage or within substrate. Quarantining plants in a separate holding environment can help prevent duckweed entering you main pond, but it’s often easier just to remove it when it starts to grow.
Is Duckweed similar to Blanket weed?
Duckweed and blanket weed are often mixed up, with duckweed being a plant and blanket weed being a form of algae. Blanket weed is often seen during summer when algae blooms are common, and usually grows alongside more common forms of algae, such as green (pea soup) algae. Blanket weed is a very fine, hair-like algae which floats on the surface of the pond in clumps; eventually covering the entire surface like a ‘blanket of weed’ – hence the name!
Since blanket weed isn’t technically a plant, it is more suitable to remove it with regular algae treatments. Unfortunately, blanket weed is far more difficult to remove in comparison to duck weed, especially if nutrient levels are high and weather conditions are right. If you do happen to have duckweed in your pond, it can actually help with the removal of blanket weed and other alage as it directly competes with them for nutrients and sunlight. Sometimes leaving a little duck weed during summer can help with algae control, especially in areas of intense sunlight.
For more information on blanket weed and how to remove it, along with algae in general, check our full guide on this here.
Do I need to remove Duck Weed from my pond?
Small amounts of duckweed may be beneficial as they can help reduce excess nutrients, control algae growth, improve oxygen levels, and even provide shade and predator protection. The problem with duckweed is that is can be difficult to maintain, and if your pond is already very high in nutrients or has problems with water quality, it can spread at lightening speed.
Unless you simply hate the look of duckweed on your pond, the only reason you would need to remove it is if the duckweed is growing too fast and causing issues with water conditions. This is especially true in ponds with fish, as excess duckweed can actually reduce oxygen levels and cause harmful spikes in ammonia. As duckweed grows a similar amount will die off, and as beneficial bacteria work to break down the decomposing matter they consume large amounts of oxygen from the water. If the amount of decaying matter becomes too much for the biological filtration, ammonia and nitrites levels will spike, and oxygen will continue to be reduced until waste is removed.
If you don’t have any fish in your pond then keeping duckweed is more a personal preference, as it shouldn’t cause any major issues with the eco-system. If you have any plants, however, such as water lilies, these can begin to suffer as duckweed competes with them for nutrients – so this needs to be taken into account if you’re considering removal.
Will Duckweed kill my pond fish?
Goldfish actually love to eat duckweed, but koi are less likely to take it unless they’re particularly hungry. Duckweed isn’t toxic to animals or wildlife, but as stated above, if duckweed growth is left unchecked you could have in-direct problems in future. Keeping a small amount of plants, which includes even duckweed, can be beneficial to fish as it provides shelter and helps reduce excess nutrients from water.
If you have a pond with goldfish and koi, duckweed should never be allowed to spread uncontrolled across the entire pond. If you’re unable to regularly maintain the pond, it is often better to just remove all instances of duckweed and hope it doesn’t come back too quickly!
Should I use herbicides (chemicals) to remove duckweed?
One of the easiest methods of removing duckweed is with chemical herbicides, which destroy the plants at the cellular level. The issue with treatments such as this is they’re extremely toxic to pond fish, plants, and wildlife. They do not distinguish between “good” and “bad” plants, and will just as readily attack any regular plants you have, such as lilies and marginal plants. We would only recommend treatment with a herbicide as a last resort, and we do not ever recommend them for fish stocked ponds as the risk of fish death will always be very high. Never use on ponds with fish!
For ponds without fish, they can be a easy and quick fix to remove duckweed, but we still strongly urge the methods below instead which help remove duckweed without chemicals. If you’ve already tried the methods below, and you have no pond fish, we recommend Tsunami DQ’s Aquatic Herbicide for maximum results. Be careful, however, as any plants you have will also be damaged so it should always be a last resort for all kinds of ponds!
The Best Ways to Remove Duckweed Naturally without Chemicals
Step 1: Test Water Quality
If you have a growing duckweed problem, you may also have an underlying issue with water quality which needs to be investigated. For example, if your pond has insufficient filtration or excess waste there may be a build-up of substances and nutrients contributing to the duckweeds rapid growth.
Testing water quality if you have fish is an important aspect of fish keeping, and should be carried out at least twice a year, or at the first sign of problems. If the duckweed has been growing very gradually, it’s likely your water quality is ok and this is it’s just natural growth rate. If the duckweed is spreading across the pond daily, you should test your water with a wide-range water test kit to make sure substance levels are within safe parameters.
The main things to look out for when testing are: ammonia, nitrites, and nitrates and phosphates. The first two are indications of insufficient filtration and can become very dangerous to fish in high doses. The latter two, nitrates and phosphates, act as natural plant fertilizers and will be readily taken up by any duckweed present in the pond. If your test comes back with a high range of any substance, this would need to be solved alongside removing the duckweed to ensure it doesn’t come back in future.
For more information on water testing and what can be done, you can read our full guide on this here. This guide includes all the information you need to reduce levels of harmful substances and how to test water correctly for the most accurate results.
Step 2: Manual DIY Removal (Net/Vacuum)
It may not be the easiest option, but the best way to remove the majority of duckweed from ponds, especially those with fish, is to simply remove it manually with a strong net or vacuum cleaner. Unlike algae, which comes in many forms and can hide throughout the entire pond, duckweed will always be clearly visible on the surface water – making it easy to net out! This also gives you full control over cleaning, and allows you to leave a little behind if you want to keep the beneficial aspects that come with small amounts of plants.
This method works best for smaller ponds, but can also be fine for larger ponds if your water quality is brought back into check. Usually duckweed will not be able to grow fast enough to require constant clean-up if water quality is good. Testing water beforehand, and improving conditions alongside manually removing the duckweed, is the best method for control and safe fish keeping.
If you have only a small duck weed problem, using a good quality net should be sufficient to clean out most of the weeds. After you have removed most of the visible duckweed, you can the move on to Step 3 and apply a natural duckweed killer to the pond, which will ensure the maximum amount of weeds are removed.
Step 3: Apply Natural Duckweed Killer
Natural duckweed treatments are basically a mix of beneficial pond bacteria which helps reduce nutrient content in the pond and starve algae and duckweed of what hey need to grow. They won’t be very effective at removing the duckweed by themselves, but they’re great if used after a manual clean-out. The benefit of using natural bacteria treatments is they’ll also help improve filtration and water conditions, making it more difficult for duckweed to return in future.
After a big duckweed clean out, adding a natural duckweed remover may be enough to stop the remaining duckweed from spreading, but we still recommend you keep on top of any further growth manually until both water quality and nutrients are in good order. Adding bacteria is more a long-term solution to duckweed, but it’s nice option for safe, wildlife-friendly weed control instead of using harsh chemicals.
Sadly, natural bacteria based duckweed treatments are a little tough to get inside the USA, and if you don’t want to order from the UK you can make use of regular beneficial bacteria as an alternative solution – although the bacteria strains are slightly different, so results may vary!
Step 4: Reduce Fish Feeding
If you have fish in your pond, either goldfish or koi, you can try reducing their feeding doses which will encourage them to eat more of the duckweed. Goldfish love duckweed more than koi, but a hungry carp isn’t going to turn down some easy meals if their stomach is growling! Many ponds with heavy fish stocks will never see problems with duckweed as the fish will eat it faster than it can grow, but if you like to feed your fish regularly they may just be too full to help out.
If you have a gradually growing duckweed problem, you can try reducing feeding to every other day to increase the amount of weeds eaten by fish. If you’re on this step after manually removing most of the weeds there was likely far too much for your fish to eat anyway, but you can still apply this step to help remove any left over plants that were missed in the process.
When you resume feeding again, you should also make sure you’re using the best quality fish feed possible, as this will help reduce fish waste. Top quality fish foods will have lower filler content (ash) and high content that fish actually need (proteins, fats, minerals etc.) in their day-to-day lives. For a full guide to the top fish feeds, check our dedicated article here.
Step 5: Remove Excess Nutrients
Finally, after removing most of the duckweed you’ll want to remove as much excess nutrients from the pond as possible. If your water tests are coming back showing high levels of nitrates and phosphates, in particular, you’ll want to start cleaning out debris and bottom muck to get these levels lower. Duckweed, as well as algae, will thrive from these substances so unless you have pond plants to compete with them they’re best removed in ponds with fish.
We recommend a few things to keep on top of excess waste and nutrient levels in ponds, all of which can be incorporated for maximum results: