Freshwater Scuds: The Ultimate Algae Eaters?

Algae is the bane of most freshwater aquarium keepers. We've all had it at some point and many of us continue to struggle with one form or another for long periods of time. No doubt algae has been the reason some have gotten out of the hobby entirely. In my case, I've had to deal with hair algae, staghorn algae, and black beard algae.

I've had success controlling staghorn algae with Seachem's Excel, but I'd rather not have to use such chemicals all of the time. I've been able to knock out hair algae using API's Algaefix, which is harmful to shrimp so it's not a perfect solution. As for black beard algae, I've found nothing more effective than hydrogen peroxide, but I've never been able to eliminate it entirely and who wants to do treatments every day?

Enter the freshwater scud. These little guys sit at the bottom of the food chain and eat decaying plant matter, left over fish food, and algae. In fact, some testing suggests that if given a healthy plant with algae, they will eat the algae and ignore the leaf. Need proof? Take a look at this before and after shot of an anubias leaf I left in a container of 50 or so scuds for 24+ hours.

Anubias Leaf Covered With AlgaeAnubias Leaf Covered With Algae
Anubias Leaf After Scud CleaningAnubias Leaf After Scud “Cleaning”
BBA in Bottle Cap: Before and 24 Hours After Introducing Scuds

Some other things I've tested:

  1. Scuds seem to be unharmed by Seachem Excel dosed up to 1.75x the standard dose. I haven't tested higher amounts so they may be able to tolerate more.
  2. The scuds cleaned off algae from Lobelia Cardinals “Dwarf” without harming the leaves. I was curious about this because these leaves are softer than those of the Anubias in the above photos.
  3. The scuds had no problem eating the algae off of wood. Once the algae was gone, the scuds moved on.
  4. Scuds will eat the “leaves” of java moss leaving the stems behind. They eat it slowly though so it's possible the moss will grow fast enough to stay ahead of them.
  5. The overnight low temperature that I've confirmed scuds can handle is 2 degrees celsius.

These are things I've read about scuds that I haven't yet verified for myself.

  1. Pygmy Sunfish are good fish for keeping a scud population in check.
  2. There doesn't seem to be a way to kill scuds that won't also kill fish and shrimp. However, some claim that using seltzer water (equivalent to a high dose of CO2) will kill scuds while not killing plants. This sounds promising since it'd be safer than using chemicals that might remain in the tank.

My hope is that I can introduce these scuds directly to my planted aquariums. However, I'm trying to find answers to the following first:

  1. Will the scud population control itself based on the amount of food available? Will the fish keep the scud numbers in check?
  2. Do scuds “prefer” certain foods over others i.e. will they eat algae and decaying plant matter first? Are there some live plants that they can't resist? There are reports that scuds will eat moss.
  3. Do scuds attack young, adult, or moulting shrimp?
  4. If needed, is there any way to eliminate scuds entirely from an aquarium?
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  1. Scuds are the cockroach of the aquarium world they will infest and eat your plants roots

  2. Ok, longish story I'll try and keep it short...

    I found 2 blue scuds living in the bottom of a small aquarium I was cycling, they must have come in a bag of small river stones from an aquarium stockist in the UK. They both disappeared never saw them in the aquarium again.

    After a few months of adding golden mountain minnows and live plants, was just about to buy some red cherry shrimp and I noticed that my aquarium had planaria, deadIy to shrimp and were killing my minnow fry, but I found a cheap and easy solution dog and cat dewormer, worked a charm also told its shrimp safe.

    As I was doing a water change into the small outdoor pond, saw blue scud zipping eveywhere! So thought I would set up a mini indoor aquarium just for them about 30 scud in a 25 litre aquarium.

    I brought in some pond water and added a small air bubbler, all was good after a few days I topped up the scuds water with some of the treated aquarium water and the scuds all died within 6-8 hours... all the flat worms survived, detritus worms and planeria died too.

    I was disappointed the blue scuds died, was gonna use them as live food in my 300 litre community aquarium but useful to know how to kill them right?

  3. Hi, from Geneva, Switzerland. Thanks for wonderfull blog on gammares. I had five or so of them living in a small 2 gallon low-tech tank with willow moss from local river. Then the tank got overrun with dugesia tigrina (water flatworm) and I believe they ate all of the scuds. Those worms love hot water and proliferated in summer.

    I was mad but, well live and learn. The scuds weren't attacking the moss but waited for carrots and cucumbers and peas, then took the spoil in their hiding place. They came out only when the food was finished.

    Now we hace moved in the town and I lost the beautiful willow moss too and currently I have a vase (1 gallon) full of Java moss, ostracodes and snails. I just introduced a couple of newly caught scuds ans found this blog, because suddenly I wasn't sûre If they won't eat up the ostracodes...

    I got banned from having a large fishtank after starting with two endlers ans finished with seven différent tanks and had to sell about three hundreds of these fish (had differwnt color hybrids) so I told myself I'll try a local biotope.
    Yes I would love to see those blue scuds too!

    • Have the scuds attacked your snails or snail eggs?…. Mine do!

  4. I have seen these around in the local creeks for years and was always tempted to set up a dedicated aquarium just for them. After reading these comments I think I will give them a try to see what they are like with plants. I have some Java Moss and other plants to experiment with and elsewhere I have read they will eat Duckweed but that plant can most certainly take over if given a chance and if they control it and Algae and leave other plants alone, sounds like a win win situation.

    • I set up a dedicated scud tank about 2 years ago...nothing huge. It is a 2.5 or 3 gal bow tank. The filter is the small sized sponge filter running with air and I got two of the small stones to keep the water clear. There is some moss from the garden and substrate at the bottom of the tank as well as rocks. I put a cutting of Pothos in there and it is fine. I also have a pothos at the top of the tank..leaves out...roots in and a couple of other plants. Some duckweed too...not a lot. I have read that putting pothos in the tank is not a good idea because it can rot...Might be true if you have large fish in the tank. I have it in my tanks with cherry shrimp, another tank with guppies and another tank with mosquito fish. These plants have been in there for over 2 years...they look great. When I clean out my filters on the other tanks, this is where I put them so they have a place of their own and do not get eaten. I know..some people might say I am nuts LOL. There are hundreds of them in the tank. They swim around all day. Personally, I like them. Their tank is very low maintenance..they are in with some pond and rams horn snails. I feed them about 1/2 tspn of ground up algae wafers once a day. Also, whatever goop I cannot take the time to separate them from when I clean out the tank goes in with them too. Between them and the snails..all gone. They also do a pretty good job of cleaning the filter of the tank they are in until I get them out. BTW..I don't buy algae wafers in assorted sizes. I put them in the blender and I get the powder I need for the scuds and cherry shrimp and the smaller chunks I need for the other tanks like the ones with the plecos and other snails.

  5. I don't recommend scuds in any tanks with plnts… they have decimated several types of plants in my planted tank (they were accidentally introduced with some new java moss) and have destroyed my large amazon swords and reduced my large amount of java moss to a tiny clump.. In my case I have had foam filters covering the entrance to my canister filter and powerhead circulator because I was protecting shrimps from being sucked in, which also kept the scuds from getting sucked in... I was not aware of the scuds until the population got quite large (I was scratching my head about what was going on with the plants)..I finally figured out what was going on after going in at night with the lights off and a flashlight and looking in the tank...Oh and they also eat the roots of floating plants...I can't treat with anything because of the shrimp... what I have done to get them under control (finally) is put a fairly coarse filter (big enough for the scuds to get through but not the shrimp) on the intake of my external canister filter and then putting a large amount of poly-fill in the canister to catch them, at this point in time I have them pretty well knocked back but still get a few hundred out of the power filter every week...Oh and they also kill snails, my Malaysian trumpet snail population has plummeted as the scuds population has gone up (another head scratcher until I found the scuds).. I would never put them in a planted tank or a shrimp tank, anything that eats scuds also will eat shrimp...I would imagine a tank without plants and some sort of aggressive fish would be fine with them (such as cichlids).

    • Thanks for sharing your experiences. Some people seem to have better luck with them i.e. no plant issues. I wonder if there are different types in the hobby with some scuds being voracious plant eaters. My own don't seem to eat much including java moss. I also wonder if it's a number of scuds vs. available food thing. Small numbers of scuds might find food more readily without having to eat plants.

      • Marios yes there are actually quite a number of sub species mentioned depending on what part of the world you live in. If yours are not attacking plants or if they are living on plants slower than what they are growing the plus points are better than negative. My Java Fern is over growing my aquarium to the point of culling being necessary with some older growth being slightly covered with algae fuzz. In another week or two I will get my culture going for a test run.

        • Their behavior is also very population dependent. I love them in my planted tanks provided there is some fish to keep their numbers reasonable. Keeps fish happy and hunting for these treats (my apistos love looking through the hairgrass for them). At this level they leave snails and moss alone.

          However, remove the fish, and if conditions are right, numbers will grow, and at a certain point they stop hiding and get really bold. Then it is like a plague of locusts (or tribbles). It changes really quick. Undergoing this in one of my tanks now. It is amazing how fast they will tear through moss in this frenzied state. Many other plants will be mowed down as well. When they get this bad, the Malaysian trumpet snails start to show up as empty shells - not sure if they are directly killed by the scuds, or if they are out-competed to death and their bodies eaten.

          Not sure about with other species, but with Apistos, they will be very happy keeping a reasonable scud population in check, but if you chuck Apistos in a tank already at the tribbles stage, they will initially gorge, but then go off scuds, so don't let it get this bad.

          Only hate them in the tanks that I have converted over to shrimp - if the tank has ever seen a scud, they are there in some small number, and will start to gain ground with only shrimp. I have a daily routine with a turkey baster to keep them in check, and plants doing great, but future shrimp tanks will be setup absolutely scud free to save myself this annoying task.

    • You are absolutely correct about scuds. Unfortunately scuds have taken over one of my Shrimp/Endler tanks. They first ate all of the algae which I thought was great. Then they ate all of the Java Moss. From there they ate the Pogostemon Stellatus Octopus and they are now munching down the Java Ferns and Anubias. So far they have not touched the Water Wisteria? Then I noticed that my Cherry Shrimp population was disappearing at an alarming rate. I started vacuuming the gravel every other days removing about twenty gallons of water to collect a few thousand scuds and I was pulling them out of the filter so often I messed up the cycle of the tank causing a bacterial bloom. Even after all of that the tank is still filled with scuds as they are boldly swimming about with the fish. I have considered abandoning the shrimp and endlers to add fish that might eat the scuds but have found no evidence of anyone having success with that either. Scuds are a dirty word around here and we are at war.

  6. Marios - what a great study of scuds. And I so appreciate the photos and details. I know this article is old yet I'm compelled to write having found it. I have a scud and snail tank for growing food for my various fish. They are fascinating little guys.

    My experience is that the population will not grow if food is insufficient. However, in a tank with fish and plants, it is unlikely that food will ever be insufficient - they will eat from the filter and between the gravel and in the nooks of the plants, finding anything they consider edible, which is a lot. If scud are added to a tank with fish, I think it is likely that the fish will eventually decimate the population unless the scuds find their way into the filter and begin reproducing there or any other place a fish literally can not swim into. I haven't tried it so I can't give any examples - just anecdotal by the way my fish scour the tank looking for them.

    The only tank of mine that has scuds and fish long term is a no tech 1 gallon fish vase with gravel capped soil that is fully planted and has been up for less than a year with only endler fish: I believe the endlers were unable to eat the largest of the scuds which have reproduced and the endlers are eating the young before they can then reproduce. My theory is that the endlers will eat all scuds that they can fit in their mouths and the scuds that are too large will die and no scuds will be left reproducing resulting in no scuds in the tank. The project will conclude when I can not find any scuds in the tank for 2 months or by 12/31/19 whichever comes first! By the way, the scuds I have are wild caught from a local river in VA and some of them are a beautiful dark blue.

    Would love to hear any news of scuds since the original post if you have any additional thoughts or experience. Sometimes we don't think to repost until someone asks for an update - and I'm sure that for every one that says great job and thanks for writing, there are many many more who read and don't comment and still appreciate it. So great job and thanks for writing about scuds Mario!

    • Thanks for sharing your experiences with scuds. I'm leaning towards the idea that they're a good addition to an aquarium since their population will be kept in check if there are fish that will go to the bottom and hunt them.

      My first bit of news is that while scuds will eat BBA, they don't seem to be good at seeking it out. That is, unless the BBA is at the bottom of the tank, the scuds won't find it and eat it. A few times I've put in leaves with BBA and if it's in the middle of the water column or at the surface, it goes untouched.

      Also, I've exposed 20+ plants to scuds along with 4 or so mosses. It doesn't look like scuds ate any of the plants or moss to an extent that it was noticeable.

      • I just put Java fern in my freshwater tank this morning and this evening I find a few scuds on the plants. I wish I could get the scuds down to a controlled population. Help.

      • I have 2 vase nano aquaria going with sand-topped dirt substrate, and they’re both full of scuds that came with plants that I bought cheaply at a local aquarium store. I wish they were blue, they would be much more fun in a nice color! Mine are white, and various browns. I had a sparkling gourami hunting something too small to see, and the population dropped, so it was probably eggs or young scuds. When she first arrived, she ate an adult scud, but she avoided them after that.

        They’re producing a LOT of mulm, and my plants are gradually diminishing.

        My big problem is that both vases have very green water. I have read that scuds eat daphnia, and daphnia eat greenwater algae, and my current theory is that this is the problem.

        The gourami died, so I haven’t gotten any new fish, because I don’t know what killed her.

        I just acquired some sand from the shore of Lake Ontario, which has been a reliable source of daphnia in the past for me, and put it in a bottle with green water that I siphoned from one vase with a super fine shrimp net over the end of the siphon. So far it’s only been a couple of days and I haven’t seen daphnia yet, but there are also no scuds showing up, so fingers crossed.

        I think I should get a betta to control them. Bettas are carnivorous and aggressive, so I expect that to work, but then I’m putting daphnia into a habitat with a Betta and scuds… they don’t stand a chance unless I can breed them in very large numbers.

        Someone with blue scuds needs to start selling them and getting them into the trade. If I’m going to have them anyway, they should have some ornamental value! Have any of you found any other interesting colors?

        • For the sand you grab from Lake Ontario, is close to the water's edge or farther away? Is the sand completely dry or still wet? Do you dig down at all? Just curious. It never occurred to me that a lake would be a good source for daphnia. I always though a smaller body of water like a pond is where they would be found.

    • I live in VA and have been trying to find scuds for quite a while. The idea of finding some that are a nice blue color would be wonderful and I have a bunch of members of an aquarium society that are interested in them also. Would you happen to remember the river and approximate location that you found the blue scuds? Would love to plan a field trip to see if I could collect some to start a culture in one of my tanks. I have a very heavy planted 75g tank with lots of algae and floting plants I'd like to see them thrive in.

      • Just saw this - I found them in North West River in south east of VA. That's not a lot to go on though as it's a huge system. Was out in canoe directly across from Bob's Fishing Hole in the areas along side of the river. There are cypress trees with mosses just below the surface and was able to drag net across them and net scuds. I don't recall time of year though as I used to go there all the time. Color ranged from tan to blue - I wondered if it was what they were feeding on that gave them the color.

        • I have scuds but nothing blue. How cool is blue!?! Would you consider shipping some to another aquarist? I have scuds in all my tanks and in my planted vases.

    • I found this thread while looking for a good way to separate scuds from duckweed. I have scuds in three tanks, a 20 community tank, where they buried themselves in the gravel, a three with Cherry shrimp, and a three with dwarf Cajun crawfish.

      In the shrimp tank, they hang out in the midwater level. The Java moss and ludwigia have to be pruned, and there's ample pond snails, so they clearly don't eat that. Jury is out on baby shrimp, which I don't see, but had problems breeding before I added scuds.

      Community tank also has wisteria and sword, no damage there either. Also plenty of snails.

      Couldn't get them going in a five with endlers, so endlers are a good control.

      In the crawfish tank there's ludwigia, hornwort, Java moss, and way too much duckweed. No plant damage. They live mostly up top in the duckweed, though some move around. This tank has only one large ramshorn snail. I've never seen them attack a snail, but suspect they go after hatchlings. But I have seen them eating the egg jelly. Plenty of baby crawfish, so no issues there.

      Do they eat algae? Hmmm. Not sure. Definitely not the algae on the glass, alas. I had to add the ramshorn because of the algae on the plants, so not convinced. What would they do without duckweed? Who knows? But I don't know what they're doing up there in the duckweed, it's certainly not keeping it under control.

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