Scuds (or side-swimmers) are small shrimp-like invertebrates belonging to the family Gammaridae. The choice of keeping or eradicating them in shrimp, fish, or planted tanks has been a popular controversial topic on online forums, and Facebook groups over the years.
On the one hand, these little bugs are amazing algae eaters and clean-up crew; they have a high tolerance of environmental conditions, high reproductive rates and can be used as fish food in our tanks. On the other hand, scuds are almost impossible to get rid of once established. In addition, their hunger has no limits that might cause some problems for shrimp, fish, and planted tanks as well.
Are you undecided whether to keep scuds in your tank? or lacking adequate knowledge of the pros/cons, whether they are safe to keep in your tanks and what they eat, all these and more will be covered in this article.
Taxonomy of Scuds
Let’s start with the fact that scuds belong to the largest animal families on earth. There are more when 10 000 scud species and this number keeps growing every year.
Scuds family is extremely versatile, they successfully conquered marine, freshwater, and even terrestrial environment. Their land-dwelling cousins are called Rolly pollies, Pill bugs, and Sowbugs
Currently, there about 2000 freshwater species. It is absolutely not possible for ordinary hobbyists to find out what kind of scud species they might have in their tanks.
Natural Habitat of Scuds
Scuds occur in saline waters, hot springs, or underground waters of caves.
Freshwater scuds colonized almost all freshwater ecosystems. They can be usually found in shallow rivers and streams with slow currents, lakes, ponds, swamps, and artificial canals.
In nature, they live in spaces between the stones, roots, or tangles of vegetation. It gives them protection against fish and other aquatic predators.
Description of Scuds
Although scuds are often called shrimp-like amphipods, it is not absolutely correct.
Unlike shrimp, they do not have a carapace, their body is completely segmented and divided into 3 groups:
- Cephalothorax (Head with 2 pairs of antennae).
- Thorax (Body with 7 segments, each segment has pair of legs).
- Abdomen (It consists of the rear 6 segments. Each segment has appendages for swimming).
How big do Scuds get?
Various scud species can grow to different sizes. However, in aquariums, we usually get species that can get to about 10 – 20 mm (0.4 – 0.8 inches) in length at maturity. Colorwise, they can be white, grey, yellow, brown reddish-black, tan, etc.
Scuds are omnivores, feeding on algae, plant, and animal material.
The traditional generalization that freshwater scuds belong to the functional feeding group of shredders. They consume fallen leaves in rivers, lakes, ponds, swamps, etc. They are responsible for the transformation of leaf litter material into fine organic matter, and their activity increases the breakdown rate of leaf litter.
Are Scuds Harmful to Shrimp?
The short answer is, yes, scuds are harmful to shrimp and should not be kept in the same tank if you are serious about breeding shrimp. Do not believe anybody who says that scuds are great for planted or shrimp tanks.
Let me explain why I have come to this conclusion. Why this question is not on my mind anymore.
- Food Competition.
Dwarf shrimp and scuds share the same resources, such as food (detritus, algae, biofilm, etc.) or space, and substantial ecological overlap make such competition even more likely. Considering the fact that scuds are extremely voracious scavengers, in small aquariums, they will outcompete shrimp easily.
Another problem is that baby shrimp will suffer the most in this situation. The point is that when shrimplets are small they prefer to stay for several days in one place where they can hide. They do not dare to swim into the open and can get only floating particles, algae, or biofilm nearby.
Scuds do not care. Adult or young they will venture into the open to find anything they can eat.
So, when you do not see as many baby shrimp as before, this is the sign of outcompeting.
- Breeding Competition
Scuds are prolific breeders. Their reproduction cycle is shorter than shrimps. Therefore, they will easily outperform the shrimp breeding eventually. The more scuds you have in your tank, the less food there will be left for the shrimp.
In nature, scuds are one of the most successful invaders in freshwater ecosystems. Some gammarid species can survive in lower oxygen concentrations, higher temperatures, pH fluctuations, salinity levels, and even out of the water for a few days!
- Potential Aggression
There is not many evidence of scuds ever eating or attacking shrimp or baby shrimp in a tank. However, it does not mean that it is not possible. Especially, during molting process, when shrimp are the most vulnerable.
As I have said earlier, there are lots of scud species and not all of them are herbivores and scavengers! Even more, in fact, some wide-spread species (Gammarus pulex, Gammarus duebeni celticus, Hyalella Azteca, ) which mostly feed on algae and fallen leaves, have been also found to act as effective predators and not only as shredders or detritus feeders. So, carnivorous and even cannibalistic behavior is not something unusual in scud’s world.
In addition, it will be a complete disaster to get Dikerogammarus villosus in any shrimp or fish tank. These scuds have earned the nickname ‘Killer shrimp’, with body size (up to 3 cm or 1.2 inches). They are voracious predators, preying upon a wide range of benthic macroinvertebrates, such as aquatic bugs, leeches, isopods, shrimp, and even small fish, dragonfly larvae, or juvenile crayfish!
You can also read “Dragonfly and Damselfly Nymphs. Monsters in Shrimp Tanks. Treatment”.
Although many people have different opinions and different experiences, there are way too many potential problems with scuds in shrimp tanks. Therefore, I do not think that it is a good idea to keep them together unless you do not care about your pets.
Are Scuds Harmful to Snails?
In the aquariums, scuds are harmful to snails as well.
I am not going to repeat how good they are at outcompeting other animals for food recourses. This time we have documented reports of how scuds attacked snails and ate their eggs.
Of course, not all snails are in direct danger. Snails with trap doors can effectively resist any attempt to bother them. However, not all snails have this kind of protection.
Note: Scuds are very bold creatures, they can also annoy snails by constantly grooming and picking up mucus. There is a possibility that it can also harm snails, even if scuds do not do that intentionally to kill them.
Are Scuds Harmful to Fish or Fish Eggs?
No, generally, scuds cannot harm your fish. On the contrary, lots of aquarists feed them to their fish. It looks like there is something in the scuds movements that trigger their hunting instinct. So, your fish never seem happier than when they are able to chase their natural food.
However, having scuds in your rearing tank can become a problem. Scuds are opportunistic, they can eat or damage fish eggs, or small fry. It is also documented in scientific studies.
Are Scuds Harmful to Plants?
Although scuds are usually compatible with a planted tank, they are still not completely plant safe. Some people say that we can reduce their appetite by feeding a lot of vegetables (lettuce, cucumber, zucchini, etc). Well, it is not completely true.
There are multiple reports that scuds started eating plants even when there was other food in the tank. They usually eat only soft parts of leaves leaving stems untouched.
Lots of aquarists noticed that Java mosses are often the first to go. During my research, I also found that scuds were responsible for eating Hornwort, Anacharis, Dwarf Sagittaria, Rotala rotundifolia, Java fern, Amazon Sword, etc.
As we can see, when they are hungry, not even the Amazon sword is hard enough for them.
Note: The soft plant tissue attracts them most. The softer it is the higher is the chance that scuds will snack on it one day.
Another way they can damage plants in the tank, then they would get down in the substrate and eat the roots off. As a result, the plants get weaker and start rotting.
It looks like only fast-growing and floating plants have decent chances to survive in planted tanks with scuds. For example, Duckweed grew steadily in a scuds tank because the plant can grow faster than they could eat it.
Where Do Scuds Get in Our Tanks?
Actually, it is very easy to accidentally introduce scuds into your tank.
They will often hitchhike on aquatic plants. In most cases, they are just too small to see and a few weeks later, you start to see scuds infestation in the tank.
Sometimes they can come even with new fish or shrimp. To be clear, in the bag from a seller.
How Do I Get Rid of Scuds In My Aquarium?
Right from the beginning, I need to say that unless it is a bare bottom tank, it is almost impossible to get them all. However, lowering the numbers by even half would help.
Yes, you have heard me right.
I have checked lots and lots of forums, I have asked people about this problem and I have also tried some of these methods myself. They are natural juggernauts – an unstoppable force to be reckoned with. This is not a joke and you will see it below.
Therefore, think twice before adding them to your planted or shrimp tanks!
Manual Removal Trap
- Place the net on the bottom of the tank.
- Drop a little piece of blanched cucumber, zucchini, green beans, carrot, cabbage, lettuce in the net. Let it sit for some time.
- Within a couple of hours, you will get a lot of scuds on that food.
- Take the net out with the food inside.
- Discard the scuds or give them to your fish.
- Rinse and repeat until you are satisfied.
The downside of this method:
- There will be always more.
- You will also catch shrimp, snails, etc. in your net.
Read my article “How to Blanch Сucumbers and Zucchini for Shrimp, Snails and Fish the Right Way”.
Although I have not seen traps designed to catch scuds specifically, we can use planaria traps (link to check the price on Amazon) for that.
This method requires some time and may work if the scuds colony is not too big yet.
- Put a tiny bit of fish or shrimp food at the bulb end of the vial.
- Lay it in dark corners where scuds prefer to be.
Introducing Scuds Eating Fish
There are many types of fish that will eat scuds.
Cichlids (like Julidochromis, Oscar, Red Devils), Loaches (like Clown loaches, Zebra Botia, Macracantha Botia loaches), Bettas, Mollies, Tetras, Pea puffers, etc. will go straight after the scuds.
The downside of this method is that:
- You have to introduce a new fish to your aquarium that you may not want in there. For example, even a small fish like Cardinal tetras or Guppies is not safe for shrimp tanks.
- In large tanks, once you have them established, even fish might not be able to wipe them out completely. At least it will take them a lot of time. I saw some reports that not even Cherry barbs, Livebearers, Loaches, and Cories could eradicate them in a 40-gallon tank (160 liters.).
- The scud population is only under control in the open space of the tank. They are always hiding in the driftwood, substrate, plants, and filter. You will not see them in the day time, but they will be there.
Unfortunately, I do not know any chemical treatment that can effectively remove scuds without harming your fish, snails, or shrimp. Scuds are tough as nails, you will definitely kill your livestock first.
There are some reports when people tried to remove scuds by heavily (10-15 times more!) overdosing Excel.
Glutaraldehyde (the active component in Excel) is a biocide that can actually cause chemical burns to tissues. To give you an idea, biocide is also used for sterilizing some medical instruments.
Excel overdosing will wipe out shrimp colony, snails, fish and even melt some plants (like Vallisneria, mosses, Subwassertang, etc.). So, you will have to remove almost everything from the tank.
However, even when some scuds survived! Remember, they can hide deep in the substrate. Stir it.
Seachem Excel – check out the price on Amazon
Carbonated Water (CO2 Nuke)
Using seltzer water, soda water, or sparkling mineral water (equivalent to a high dose of CO2) can kill scuds while not killing plants. It should suffocate them and cause a large PH swing.
- Lower the water volume as much as you can. The more water you can drain out of the tank, the more effective the treatment can be.
- Add carbonated water. It should completely cover the substrate where the scuds are hiding.
The downside of this method is that:
- Very time-consuming.
- It will kill fish, shrimp, or snails. They have to be moved to another tank.
- Some scuds can still survive.
There is no need to explain, that Copper is very dangerous in fish and shrimp keeping.
All crustaceans, like dwarf shrimp, crabs, crayfish are especially sensitive to it.
You can read more about it in my article “How Copper Affects Dwarf Shrimp”.
- Buy a copper test kit.
- Lower the water volume as much as you can. The more water you can drain out of the tank, the more effective the treatment can be.
- Add Copper sulfate. The concentration should be at least 1.5 – 2 mg/l (or 5 – 2 ppm).
- Leave the tank for 3 days.
- Fill the tank and do a large water change.
- Fill the tank again and test the copper level.
- You need to remove copper before adding fish or shrimp back.
The downside of this method is that:
- Any residual traces of copper can still kill the dwarf shrimp when you decide to move them back.
Some people suggested that AlgaeFix should work against scuds. However, I could not find anybody who really tried it.
In any case, AlgaeFix has a specific note – do NOT use AlgaeFix with crustaceans, including crabs, shrimp, freshwater shrimp, and freshwater lobsters.
Well, this is what we need. However, do not think that AlgaeFix overdose is safe for fish or snails. Remove them.
AlgaeFIX – check out the price on Amazon
The Nuclear Option
Many shrimp and fish keepers go with the nuclear option. This is the only truly effective way to remove scuds.
- Evacuate all your shrimp, fish, snails into another bare bottom tank (quarantine tank).
- Visually check that no scuds were transferred.
- Do a tear down.
- Scrub and clean your tanks, decorations, and driftwood with Hydrogen peroxide (link to check the price on Amazon).
- Leave them in the sun to dry for a few days.
- Throw away the sponge (Replace the filter media).
- Quarantine plants.
- Quarantine shrimp.
- Ideally, you should never re-use substrates from an infected tank. If you still think of using it – boil it or pour boiling water on the substrate straight from a kettle. Stir the substrate to kill everything in it.
How Do You Raise Scuds?
Scuds are excellent feeders for fish and dwarf frogs. Besides that, they can live and thrive anywhere. Therefore, if you decided to culture them, it will not cost you much.
- Tank size. I would say that there are no minimum requirements. However, it will be easier if you have 2.5 – 5 gallon tanks (10 – 20 liters).
- Filtration. Scuds are tiny creatures. Like shrimp, they do not produce a lot of waste, therefore, you do not need strong filtration. Large pore sponge filters will be the best choice. They are cheap, easy to maintain and clean, provide a lot of surface to graze on and live in!
- Aeration. Minimal. No current, just a few bubbles every 1 or 2 seconds apart.
- Substrate. Any type will suffice. However, coarse gravel will provide more hiding places; it will make them happier.
- Driftwood and décor. The more you have the better. It will provide more feeding areas and hide places.
- Plants. Again, the more you have the better. It will provide more feeding areas and hide places. Keep in mind, scuds can eat plants when they are hungry. Therefore, do not add plants you want to keep safe.
- Lighting. Light is not important for the scuds. Lighting should be adapted to the needs of plants and algae in your tank.
- Water Parameters. Scuds are extremely hardy but for best results:
Temperature: The optimal temperature should be in the range of 20 – 24 °C (68 – 74 °F).
pH: Optimal water pH should be in the range of 7.0 – 8.0.
Hardness: Scuds will appreciate hard water GH >10.
- Calcium. Add crushed corals, cuttlefish bones, etc. They need calcium for their shells (read more here).
- Feeding. A few times weekly. Do not give them too much, only in small amounts. Leftovers can spoil the water. Fish, shrimp or crab food, algae wafers, blanched vegetables, etc.
- Maintenance. Check your water parameters and do regular water changes (10 – 20 %) to keep it optimal. Use only dechlorinated water for scuds tank setup.
Tip: Scuds do really well in green water.
As you can see, scuds are even less demanding than Neocaridina shrimp.
Read more about “Basic Shrimp Tank Setup for Neocaridina.”.
How Fast Do Scuds Reproduce?
It really depends on the species. However, if we are talking about Hyalella Azteca (one of the most popular scuds for culturing in freshwater tanks), they reach maturity at 3 weeks old (after their 6th molt). Female scuds are smaller than males.
Scuds mate when female molts. Adult males have an intermoult period of 20 days (range 8 – 43 days) and females 11 days (10 – 13 days).
Prior to molting, males attach to the females from above using their legs. It has been observed females carrying the males on their back for days before they molt. During molting, males help females to shed the old exoskeleton.
After fertilization, they immediately leave females.
The incubation period lasts around 11 days at 20°C (68F). Changes in temperature affect the metabolic efficiency of the scuds. The metabolic rates increase with increasing temperature.
Female scuds can usually carry up to 20 – 30 eggs. Big females can have even more eggs.
After hatching, the young of scuds are born as tiny copies of the adults — not more than 1 mm in length and are perfectly independent.
How to Prevent Scud Infestation?
The best way to keep scuds out of your tanks is to head off the problem before it takes hold.
Therefore, always quarantine everything! Treat everything as if it is infected. I seriously mean it.
Fish, shrimp, crayfish, crabs, snails, plants, driftwood, decorations, etc. – anything coming from another source that may have been in contact with any biological material.
If you practice an extended quarantine you will notice that there are scuds and can deal with them.
Note: Some people say that another way to control scuds is to stop overfeeding. Although I do agree that overfeeding is a bad practice and should not take place in the aquarium hobby, I still have to say that it will not work against scuds. If you feed your shrimp, snails, or fish, these little buggers will always find food in the tank. In addition, according to some experiments, some scud species survived without food for almost 10 days!
To find out more, read my articles:
How to Quarantine and Disinfect Aquarium Plants.
How To Setup an Easy Quarantine Tank
Lots of times you may see that scuds are harmless creatures that can hitchhike on aquatic plants in our aquariums. Well, it can be true only for fish tanks where fish can eat them. Indeed, it is really nice to have some biodiversity in the tanks.
However, scud infestation in shrimp and planted tanks can turn them into nightmare. Dwarf shrimp cannot compete, hurt or even outbreed them while scuds can.
Do not get me wrong, scuds are a truly amazing cleanup crew. They eat algae in no time and can be a great addition for tanks where you can control their population. Otherwise, their hunger is so great that they can become a real problem.
18 thoughts on “Scuds In Shrimp And Fish Tank”
i have an infestation in my qt tank, i dont have anything to feed, should I just take all possible nutrients out and wait for them to starve? I feel kinda bad lmaoo
Do you want to remove them from the qt tank?
Starving them can take a lot of time!
If there are algae, biofilm, etc. in the tank, some of them may survive for a VERY long time.
I’ve found after some reading, basic water changes, a few a week to catch as many scuds as possible. Without hurting your nitrogen cycle. I found while treating for cotton mouth that the scuds scatter with Primafix treatment, I’ve noticed with observations it must wreck some havoc on their breeding cycle. I’m about 4 was in and have cut the scud problem down to only a handful. Will update in another month.
It will be interesting to know the results.
Would a bleach or hydrogen peroxide dip kill scuds? I just bought some plants from my LFS and did an alum soak for about six hours. When I rinsed the plants, I discovered a few critters which I think after asking and looking online, are scuds. I haven’t put the plants in a tank yet as I want to do as much as I can to remove the scuds. I can quarantine them but wondered what else I could do to head off a potential scud infestation.
Bleach should kill scuds, if they do not have anywhere to hide. Nonetheless, they are extremely hardy creatures, therefore, I’d quarantine plants as well. Just to be sure.
Zebra danios will eat them but as stated after a couple of weeks you might think they are gone but are only controlled to the point you don’t see any take the danios out and withing a couple of week your infested again I had scuds in a shrimp tank and I swapped the shrimp and danios over and I thought the danios had done the job but the danios spawned and I took the adults out to raise the fry and within two weeks the tank was infested again I’m waiting now till the danio fry are big enough to eat them again….. Plus when moving my shrimp o also moved 4 scuds by accident so now I have two infested tanks bummer
Hi Peter Ohare,
Yeap, scuds are true survivors. It is almost impossible to get rid of them once they settle.
I have been in your shoes and I feel your pain.
I just totally nuked a tank with co2 to kill a scud infestation. I removed all the shrimp first to a quarantine tank then just let the co2 blaze away for about 10-12 hours. It killed all the scuds and even killed all the limpets that were everywhere as well. But I ultimately did a whole teardown because there were so many carcases it fouled the water. I am just wondering if I can reuse the filter media, I let it sit out in the 100 degree heat for three days do you think all the scuds and eggs are dead? Or all new media for the filter?
Thank you for the feedback!
I’ve got a scud infestation in the 20L I’m using for breeding CRS. I’ve got two berried females who should be producing shrimplets in a week or two. What is your experience? Will the presence of scuds affect the birthing process? Will they affect the new shrimplets?
Thanks for all advice.
Hi Doug Martin,
In the long run, scuds will affect your shrimplets. They will outcompete them for food thus lowering the survival rate.
Personally, I have not seen direct agression, unlike, some friends of mine.
Hi, I’m trying to set up a 20g long with a pea puffer and some scarlet badis. Will the pea puffer be able to keep the Scuds in check if I add them in as a sort of self sustaining food source, or will they still get out of control?
Nobody will tell you for sure because it depends on your tank set up and many other factors.
For example, if your have a deep substrate with a lot of places to hide (driftwood, stones, decor, plants, etc.) and scuds had some time to settle, in this cases, it is possible that your pea puffer won’t be able to catch all of them.
If I have a tank with 2 African dwarf frogs, 4 Khuli loaches, and 5 Thai nano crabs, would it be safe to introduce scuds to my tank? I have a couple snails in there and I want to make sure that the scuds population will stay low enough so that they don’t bother my snails. Although I feed my snails well, I’m just not sure the scuds population would be kept in check.
Love your website so much!
Hi Olivia Cloud,
I highly recommend that you avoid doing this. Managing the population of scuds in a tank is almost an unrealistic expectation.
If they become established, they will be difficult to get rid of if necessary.
Ultimately, the decision is yours, but I suggest refraining from doing this.
I have ONE scud. In my 3 gallon neo shrimp tank. Will ONE become really bad? I just had a cherry have shrimplets
I’m just curious, how did he end up in your aquarium?! Are you sure he’s alone? Anyway, no, one scud won’t be a problem for you.