An animal with a name enough to make most peoples’ skin crawl, and one that can quickly become a concern in garden ponds – the leech!
Although unpleasant for humans, most leeches are actually harmless to ponds so long as their populations are controlled. Only a few species actually suck blood, and most fish, especially koi, love to eat them as a snack! Even so, certain species of leech are more dangerous to fish than others, and it’s not always easy to determine the best course of action if you find a leech swimming around your waters.
In this article, we cover some common questions regarding pond leeches, as well as list the best leech removal methods and how you can help prevent future leeches from calling your pond their new home.
Pond Leech Facts
Where Do Pond Leeches Come From?
Finding leeches in your pond isn’t common, and you can go an entire lifetime without ever seeing them when visiting your waters. Depending on your location, the climate, and where you purchase your pond supplies, you may never have to worry about these creepy crawlies.
Although leeches can be found naturally in various locations, the single most common way a leech will find its way into your pond is by hitching a ride on plants, fish, or rocks you introduce to your system. Leech eggs are tiny, and can go hidden within the substrate of pond plants or become attached to new fish you add over time. Pond plants are often the biggest culprit for bringing leeches to ponds, but luckily, these kind of leeches are not usually the bloodsucking variety and instead feed on the bottom sludge and organic matter on the pond floor. Even so, the added bio-load of an increasing leech population, whether they’re thirsty for blood or not, can cause all sorts of issues on the eco-system if left unchecked.
The best way to prevent leeches from arriving is to fully inspect new pond supplies you buy, and even quarantining plants and new fish before introducing them to your main system. Adult leeches are easy to spot, but eggs aren’t always that easy and their hatching time is different for each species. Quarantining new arrivals for 2-4 weeks is good practice, and as many common leech species have fairly short incubation periods, you may be able to catch the hatching offspring before they’re added to the main pond.
What Do Pond Leeches Eat? Are All Blood Suckers?
Most of us will associate leeches with the bloodsucking variety also used historically in medicine for removing blood and treating a variety of illness. Although you can still get bloodsuckers in your pond, and some are even specifically evolved for fish, the most common type are plant leeches which feed primarily off bottom sludge and organic waste. The reason these are the most common is due to the fact that pond plants are easiest way for leeches to get into the pond in the first place; and where you have plants, you have plant leeches! These types of leeches will happily feed away at the bottom sludge and dying plant matter in the pond, often hiding away in substrate as they move about the pond liner.
Other types of leeches, which actually account for most leech species, are predatory in nature and will hunt other small invertebrates, such as snails and slugs. These can also find their way into your pond, although they’re less common in comparison to their vegetarian cousins. Predatory leeches can arrive the same way as plant leeches, and although they won’t harm your fish, they’ll certainly reduce your natural snail, slug, and worm populations over time.
The final type, and the one we all know best, are the bloodsucking variety of leeches. This sub-group of parasitic leech feed directly from the skin by attaching themselves to their host with small hooks around the mouth. They secret a substance to numb the pain receptors, and produce toxins to prevent blood from clotting. Although unpleasant, most blood-sucking leeches can’t survive in ponds for long, but there is a special species which can, and this is the one to watch out for if you have koi or goldfish!
Are All Pond Leeches Harmful to Fish?
Plant leeches and predatory leeches are not harmful to pond fish, and not all blood sucking species are actually capable of using fish as their hosts. With that said, there is one species of leech to watch out for if you have goldfish and koi, and that is the fish leech (piscicola geometra).
These long worm-like leeches are capable of attaching themselves to fish and sucking blood from their host, which can lead to dangerous infections and stress. The adult leeches are around 2.5 cm long, and often arrive to the pond attached to fish or inside pond plants. They’re difficult to catch as they can attach themselves to any part of the fish to feed, and often like to hide away within the gills and under the fins. The leeches, although unpleasant for the fish, are not deadly by themselves. The problem is the open wounds they leave which can lead to nasty infections, both viral and bacterial.
In terms of a leeches’ life cycle, this varies depending on the species, but the most common fish leech, piscicola geometra, has a 30 day life cycle. This relatively short cycle can be used to our advantage to help starve the leech and prevent the eggs from hatching and finding a meal – hence why a quarantine period is very effective.
How do I know if I have blood sucking leeches or not?
A simple test to see if you have hungry blood suckers or friendly vegetarians is to tie a small piece of raw meat, such as liver, to string and place this in your pond close to the surface. As the smell of the meat spreads, any blood sucking leeches you have will be attracted to it and start swimming over. If there are no worms after a few hours, it’s safe to assume you have plant leeches feeding on bottom muck. If you find the piece of meat has become an active feast after a few hours, then you have the blood sucking variety. An alternative to the string method is to make use of a professional pest trap, such as the one we reviewed below in our treatments section!
Best Pond Treatments for Leeches
Step 1) Reduce Bottom Sludge/Muck
- Max; Suction Depth: 7 Ft
- Suction Hose Length: 16 Ft
- Discharge Hose Length: 8 Ft, Max; Flow Rate: 1300 Gph
If you have plant leeches they’ll be feeding from your bottom sludge, as well as laying their eggs here and raising offspring. Even blood sucking leeches can be controlled to some degree by cleaning muck, as most all leech eggs will end up at the bottom in the sludge, even if they don’t feed from it.
If you have fish in your pond, such as koi, cleaning sludge has a range of benefits, such as helping to improve water quality, clarity, and control other parasites and pests. Sludge builds up as natural debris falls into the pond and sinks over time, which includes leaves, twigs, and even dead insects. As it sinks to the bottom is begins to decompose, it acts as a natural fertilizer to plants, but also raises ammonia and nitrite levels which beneficial bacteria need to break down.
All leeches love to hide in sludge and lay their eggs there as it’s hidden away, dark, and warmer than the upper levels of water. Whatever type of leech you have can probably be reduced by keeping the pond liner clean until the parasite has been culled or starved. The best way to clean a pond liner is with a combination of pond vacuum cleaner and natural sludge eater product. For smaller ponds you could simply use a wide-mouth net to scrap the bottom of sludge, but for larger ponds we recommend investing in a quality vacuum cleaner. A strong vac won’t just remove the sludge where leeches hide, but will also suck up any free-swimming worms trying to wriggle away. After cleaning sludge with a vacuum, you can supplement with a sludge eating bacteria product to help reduce any left over traces of sludge which may become food for hatching leeches.
Step 2) Add Leech Traps to the Pond
Although designed for Aquariums, quality underwater traps can also be used in ponds to catch a variety of pests, including leeches. Almost all traps work with a similar mechanism, providing bait which encourages pests to enter a chamber which they can’t escape. The problem with choosing a leech trap is many are designed for very small scale use, but the Pest Trap (pictured) from Choice Aquatics can also be used for ponds to catch leeches and other bloodsucking insects.
This trap measures 6.5″ in length and features two tunnels for catching both worms, snails, and leech. The tunnels are designed for easy entry and no escape once entered. Bait is placed in the main chamber and pests will enter through one of the two tunnels depending on the species. You’ll have to empty the trap once a day of leeches, and it should be placed on a shallow pond shelve for best results. If you also catch friendly snails and pond insects, you can simply throw these back into your pond after removing the leeches.
Although you can use any kind of meaty bait in the trap, we’ve always found a bit of chopped liver is the best leech attractant. You don’t need much, as even a small amount will attract leeches from all over the pond as the taste travels through the water.
If you have blood sucking pests, you should start catching them within 24 hours of placing the trap. If you haven’t caught anything after a few days you can try a different bait, but this could also mean you don’t have any more biting leeches around (pond vacuum may have been enough!).
Step 3) Remove Adult Leeches from Fish
- Contains one (1) API POND MELAFIX Pond Fish Infection Remedy 64-Ounce Bottle
- Heals infections and repairs damaged fins, ulcers and open wounds
- Contains natural, botanical tea tree extract to quickly and rapidly help fish
After cleaning out your bottom muck, you’ll also want to check over fish for any adult leeches already attached to them. Although they could attach themselves anywhere, you’ll commonly see leeches around the gills and under the fins/stomach of fish as this is where they’ll get an easier meal. You can check over goldfish individually by using a basic pond net and carefully guiding them to the surface. Since koi are much larger, we recommend a specialised sock net designed for handling larger pond fish. You can use this net to gently guide your koi to the surface, and you simply close off the open end and lift the koi to inspect them in the net. If you see any pesky leeches, you can use regular tweezers to pull them off carefully.
After inspecting and removing any leeches, you should treat the pond with an antibacterial treatment which will help prevent any infections occurring. These kind of treatments will also promote faster wound healing, so they’re especially useful after removing leeches from the scales.
Fish can be added back to the pond alongside the treatment and should be carefully monitored for a few days to make sure they’re recovering well. You should also perform daily inspections during this period to make sure no further leeches are causing problems for your fish.
Step 4) Quarantine New Plants & Fish
If you want to prevent leeches in future the best way is to quarantine any new fish, plants, or ornaments you add to your pond system. A 2-4 week quarantine is optimal as this falls in line with most aquatic leeches egg incubation periods so you can catch young worms as they hatch. Failing this, a week of quarantine is certainly better than no quarantine, and is always good practice for safe guarding your pond. Not only will this help you identify leeches, it will also help prevent parasites, such as flukes, and stop sick fish spreading new illness to healthy fish in the pond.
Aquatic plants can be quarantined in a submerged container or aquarium tank within their purchased substrate. They should be fine in this environment for a few weeks so long as they’re getting some sunlight. Goldfish and other small fish can be added to a large aquarium tank, which also makes is easier to see any problems (or hitch-hikers) they may be carrying with them.
For ornamental koi it’s a bit more tricky as they’re such large fish, but if you’re serious about combating parasites and leeches you should consider investing in a dedicated quarantine tank for new arrivals. This could simply be a preformed raised pond, or if you’re quarantining several large koi, you can invest in a specialised tank often used by koi breeders, and also called a “show tank” (example pictured).
Although it may seem like a hassle, quarantining plants and fish is the single best way to prevent leeches and other parasites getting into a healthy pond. As the saying goes, “prevention is always better than cure” – it’s easier to stop leeches from getting in compared to removing them when they’ve made the pond their home.
Excellent and well presented information. Thank You
Hi Carol,
Thank you for the comment! It’s great to know the article was helpful to you.
Hi I have just cleared an old well and built it up out the ground it has a drain from somewhere and constantly drips filling the well , we now have what I think is a leach looks a bit like an eel about 8-10 inches long when swimming like a snake any ideas what type it is ?
Hi David,
That sounds a little big to be a leech! Whereabouts are you located? Does it have any particular color/pattern/features? Do you have any pictures?
Our lake is 12 acres and 20 feet deep in the deepest spot. Leeches are seen mostly in the beach area but are probably all over. How do you recommend we get rid of them in the beach area. It was stocked with bass and bluegill approx. 50 years ago.
Hi William,
The presence of a large number of leeches usually means there is an imbalance in the food chain and the leeches lack natural predators. Turtles, crayfish, waterfowl and most fish species will eat leeches to some degree, so a long-term solution could be to slowly reintroduce a predator that can help control the population. Redear sunfish, in particular, will aggressively hunt and eat leeches, so they may work well as a control solution if your water parameters and climate are suitable for them.
hello, I live in Muskoka Ontario and we have a beaver lake that feeds multiple ponds on our property as the small rivers go downhill towards many small lakes. We have six inch long leaches everywhere that I assume will bite. We tried salt blocks but they don’t seem to care at all and swim right through the salt. Any ideas?….John
Hi John,
It sounds like you have quite an established leech population! In your case, particularly since these are natural ponds fed by a lake/river, I think your best bet may be to introduce some leech predators. Leech eggs are incredibly small, and there are likely a great many of them since there are so many large leeches actively swimming about. You can’t really just remove the entire bottom sludge layer since these are natural ponds (though you could remove some and it would likely help). Turtles and crayfish eat leeches readily, as do most fish (bass, catfish, and redear sunfish in particular will actually seek out leeches) and some waterfowl species (particularly ducks). You can also introduce beneficial bacteria into the ponds, as these will help break down the sludge layer, which leeches thrive in. Leeches also really thrive in poor water quality, so increasing aeration will help deter them as well – you could try installing aerators in each of the ponds, and introducing native submerged plants to help oxygenate and clean the water.
I want to also mention that not all leeches prey on people. Actually, I believe only one species in Canada will readily attach to people – Macrobdella decors. The vast majority of leech species aren’t bloodsucking, and instead feed on aquatic insects and decaying plant matter. So you might be just fine to swim with those leeches, though I quite don’t blame you if you’d rather not!
where do I get red eared sunfish?
Hi Debbie,
Whereabouts are you located? They’re quite unusual to see in the hobby, so you’d likely need to speak to a local store or koi/pond dealer and see if they could possibly order some in or put you in touch with a specialised dealer.
Hi, Can i say the info i have learned is second to none.
i have leeches in my pond filtration pipes and i know where they came from,( my friend gave me some yellow flag iris he got from a wet area in the local lammermuir hills in scotland.
Hi George,
Thank you for the kind words! Very happy to hear you found the information helpful.
Yes, leeches often arrive from external sources, usually piggy-backing on plants, ornaments, and even fish! They don’t usually cause huge problems, but they’re definitely much easier to deal with when their numbers are low, so setting a few traps with meaty bait can sometimes remove them before they can establish themselves.
what about leeches that are in the soil, particularly in the grass and plants of shady areas…where moisture is higher and not dry? is there are a natural predator on land for land based leeches?
Hi Chris,
Simply placing leech traps with meaty bait and replacing them regularly could help reduce numbers, but I’m honestly not sure of any other direct method (or predator) which could fully tackle territorial/semi-territorial species. One idea is to install a few bird houses and water baths around the area, as most birds will happily eat leeches, even if they’re unlikely to remove the whole population.
Hello. I have a small garden pond ( about 8 foot across and three feet deep). I have a huge leech problem. I have thousands of them. All about 1cm long. They are clogging up my external filter and the pipework to t he filter even though it is fast flowing water. They seem to breed in the filter. They don’t seem to be harming the goldfish but I do need to control them. I also have newts so I am reluctant to hoover the bottom of the pond. Is there a chemical cure I can use to get rod of them?
Thanks
Hi Simon,
That is certainly a lot of leeches! First and foremost, leeches are not only pollution-tolerant, but thrive in poor water quality with excess nutrients and decreased oxygen, so the very first thing that you should do is test your water quality and figure out your parameters. This does depend somewhat on the leech species (more info can be found by consulting the figures in this research article: https://www.researchgate.net/publication/322400262_Assessing_the_sensitivity_of_leeches_as_indicators_of_water_quality/figures), but as a general rule of thumb, leeches point to something being wonky with water quality.
Chemical approaches should be your last resort, so first try getting rid of the leeches manually. That is, you can get some sort of container, punch holes in it just large enough for the leeches to get through, bait it with raw chicken or some other form of raw meat, put the lid on top (to prevent other predators from getting into it) and place it in a shallow area in your pond. The leeches will sense the meat, and after feeding will be unable to get back through the holes. You can repeat this as often as needed, refreshing with fresh meat each time. Commercial leech traps can also be found online and in some pond retail stores.
Leeches generally lay their eggs and develop in muck at the bottoms of ponds, lakes, and slow-moving rivers. If there is indeed a mucky layer at the bottom of your pond, try clearing this out with a pond vacuum and replacing with fresh substrate, although it’s understandable that you’d rather not do this given the presence of newts! I appreciate you not wanting to harm or disturb them! There are some products available that help to reduce and prevent the further development of pond sludge. This article reviews some of the best sludge removers that we’ve come across: https://pondinformer.com/best-pond-sludge-remover/
Hi
We have a middle size pond in the garden with only good old fashioned goldfish. Some years ago we had a large catfish get into the pond eating all but 1 goldfish . this one goldfish is a pale pink, we call her Pinkie (original name I know). Recently we have spotted a couple of small orange spots on her back. Is this some sort of problem and if so what should we do about her, we think she is a girl as she keeps increasing in size, hides, and then pops out along with a few black gold fish a short while later. What a gal!
Hi Phil,
These sound like the beginnings of ulcers, and could be caused by a number of things: bacteria, parasites, fungus. All are ultimately related to water quality. I would test your water quality to see what comes up, and also perform a water change while also treating with something like MinnFinn. It’s a natural and biodegradable broad-spectrum treatment that is able to treat bacteria, parasites, and fungi. It’s also not horribly concentrated, so the risk of harming Pinkie should be low so long as you dose correctly. It could also be the start of ammonia poisoning, as that can manifest itself as red-orange spots on fish. Definitely test for ammonia and chlorine levels while testing your pond!
If none of the above approaches yield anything, it’s possible that your fish is just experiencing some natural color change as she ages. Certainly keep an eye on her, and let us know if she develops any other symptoms/change in behavior or appearance.
Hope this helps, and best of luck to you and Pinkie!
Hello again. I posted earlier and thought I should add that our ponds are hundreds of yards long with a deep area where we have been swimming. We don’t stay in the water for more than thirty seconds at a time. Just enough time to cool down, but the big six inch long leaches are everywhere. They seem to swim along the surface like snakes and are sort of flat and wide as opposed to round like a snake.
Great site, tons of info. My question is, my filter system seems to get clogged more and more often (bio ball type) and I have noticed I get huge chunks of debris now when I set the filter to “clean” mode. Also noticed leeches in the debris. Is there any link between the leeches and this sudden increase in frequency of “clean” cycles and increased debris? Thanks
Great article thank you. One of my large White koi has 2 leeches attached to the top of its back and the scales there are therefore reddish. It had a leech in the same place a couple of weeks ago and the scale stayed reddish. I’m concerned it will get infected. The neglected fish pond we inherited (moved in October 2018) is huge, about 40 metres by 30 metres. It has a lot of sludge on the bottom and plants that need removing. My questions are 1) can you recommend a really good sludge vacuum for a large pond? and 2) should I treat the koi with anything specific ( it will be very hard to catch it). Once the sludge and excessive plants are removed should I treat the water?
Is there a professional body who could come and give advice? We live in BH22.
Many thanks.
Hi Kate,
I’m sorry to hear about your koi! That’s a tough situation, especially as your pond is so large.
In terms of vacuum cleaners, the best one for that size of pond (due to its large hose length and reach) would probably be the Matala Power-Cyclone. It’s quite a heavy unit but the additional reach and large capacity makes it more efficient for larger water bodies. Unfortunately, it’s also quite popular and often out of stock from 3rd party retailers, but you may be able to find one via the manufacturer’s website.
Alternatively, you also have the Pondovac 4 or 5 models – both of which have less reach but have more features and are easier (in my opinion) to handle. I’ve personally used the Pondovac 4 for years and years and it has never failed me. Although I don’t tend to work with many large ponds, I can’t see it having any problem with sludge unless your pond is also extremely deep. You can view my recommendations in full here: https://pondinformer.com/best-pond-vacuum-cleaner/
In regards to the koi with leeches, I feel there is not much you can do here in terms of pond-wide treatments without also risking serious disruption to the eco-system. Even with much smaller ponds, it’s always recommended to isolate and quarantine a sick fish and treat them separately. This ensures other fish are not stressed from harsh treatments and the natural eco-system is kept healthy.
Do you feed your koi with pellets? Even if you don’t, I’d actually recommend trying to catch this koi with a net by enticing it with some food and treating the koi in a holding tank (Rubbermaid stock tanks would be fine for this and are widely available, for example).
If the koi isn’t healing, you do run the risk of secondary infections which may require anti-biotics. These need to be used in an isolated environment as they just would be more damaging than beneficial on such a large scale. Here is a guide on the treatment of ulcers/infections in koi, as there is a section on wide-spectrum anti-biotics at the bottom: https://pondinformer.com/koi-carp-ulcer-treatment-guide/
If you don’t feel comfortable treating yourself, you could also get a professional veterinarian to come take a look (although this would still require catching the fish!).
If the koi hasn’t had the leeches a long time and the wound doesn’t look to be worsening, you could also just give it time and see if things start to heal. If the koi is eating, swimming and not acting strangely, they’re unlikely too stressed at this point and still fairly strong – which is a good sign!
Finally, if your water quality parameters are showing higher than normal ammonia/nitrates when you run a water quality test you can perform a 25-50% water change after the pond clean. It would probably be a good idea to keep an eye on parameters after removing the plants/sludge to ensure things remain stable in future. You can find some info on test kits here: https://pondinformer.com/best-pond-water-test-kit/
In any case, a tough situation! Whatever happens, I really wish the best for your koi.
Hi our club has an approximately 2.5 acre swimming lake that gets filled up in spring by main lake and drained in fall back into main lake. We have found some leaches and want to know how to effectively treat this as we have been using muck pellets for years and we also dont want to harm swimmers or the main lake.
Thank you
Hi Lisa,
Getting rid of leeches in that large of a lake is going to be quite the task! Additionally, not all leeches are bad – in fact, most don’t even suck blood and only feed on plants, muck, and detritus, thus helping the ecosystem. Have these leeches latched onto people or fish that you’re aware of? You can also test this out by placing a piece of raw meat into the water and checking on it after a few hours – if leeches are attached, you have the harmful kind.
As for getting rid of them, unfortunately I don’t think that’s possible for such a large water body that also shares water with another lake! With ponds, you can drain them completely and treat the leeches or wait for the adults and eggs to dry out and die, but this isn’t really possible with lakes without harming everything else and devastating the ecosystem. Your best bet is to add in a good deal of beneficial bacteria to help eat through the muck that leeches typically hide in, and also employ leech traps to help reduce their numbers, particularly in the areas that are heavily used for swimming.
I hope that this helps!
Hi, I’ve found all these comments & advice very informative & helpful, so thank you for that. I have a 160litre pond & leeches living quite nicely with a lot of guppy fish. I always thought that the little red worms I found in filter sponges, were earthworms, for some strange reason. Handling anything now becomes a worry for me. I assume the dead leeches just go in the rubbish? After reading these comments, I think my best avenue of attack will be setting a trap firstly, then onwards from there. When I did separate some leeches previously, I ended up pouring boiling water over them & was stunned by all the babies, I assume, that lined the circumference of container.
Great interesting site.
I have a 5000 litre pond and fish leeches. Tried drying it out but they are back. Would a high volume pump and a micro mesh filter help control the numbers ? Or am I onto a loser here. Norfolk UK.
Cheers Richard
We have a swimming pond in our backyard and now have a leech problem. The pond is about 70’ x 40’ and is zero entry to about 5’ in the middle. We currently don’t have any fish. This year we have 100s of small leeches in the shallow waters. I am assuming they came in on plants and aren’t blood sucking as they are reproducing and outside of frogs we don’t have anything for them to stick to. With the bait bucket with meat work for vegetarian leeches?
Hi Jodi,
I’m sorry to hear about your leech problem. I don’t think the bait bucket with meat will work for non-blood sucking leeches, as they are not attracted to animal flesh. You may want to try some other methods, such as adding salt to the water, introducing fish that eat leeches, or removing the plants that they hide in. You can also use a net or a rake to manually remove them from the pond. I hope this helps and you can enjoy your swimming pond again soon.
https://pondinformer.com/best-pond-lake-rake/
https://pondinformer.com/killing-lake-weeds-with-salt/