Quick answer: Scuds are tiny freshwater amphipods, little comma-shaped crustaceans that look like miniature shrimp and dart around your substrate and plants. Here is the honest verdict: in most tanks they are a good thing, not a pest. They are a free cleanup crew that eats detritus and waste, and they are excellent high-protein live food your fish will happily hunt. They only become a problem in specific setups, mainly dedicated shrimp tanks, fry tanks, and overfed aquariums where their numbers explode. So whether you keep them or remove them depends entirely on what else is in your tank.
What are scuds?
Scuds are small freshwater crustaceans in the order Amphipoda, usually from the genus Gammarus (the larger, fast-breeding ones) or Hyalella (the tiny ones that hitchhike on plants). People call them scuds, side-swimmers, or Gammarus shrimp.
They look like tiny curled shrimp, comma or C-shaped, grey, brown, or slightly translucent, and most are only a few millimeters long, though some Gammarus reach around 8 mm. Their giveaway is how they move: they swim on their side, which is why they are nicknamed side-swimmers, and they are fast, darting for cover and burying into the substrate or moss. You will often notice them most at lights-out, scooting along the glass.
How to tell scuds from copepods, seed shrimp, and planaria
Scuds are one of several tiny critters that show up unannounced, and it helps to know which one you have.
| Critter | Looks like | Moves like |
|---|---|---|
| Scuds | A small comma or C-shape, like a curled mini shrimp, grey-brown | Swims on its side, fast, darts and hides |
| Copepods | Much smaller white or clear dots with a teardrop shape | Jerky, darting little hops |
| Seed shrimp (ostracods) | Tiny round or bean-shaped “seeds” with a hard shell | Zip around erratically |
| Planaria | A flat worm with a triangular, arrow-shaped head | Glides smoothly along the glass |
If it curls like a shrimp and swims on its side, it is a scud. For the worm-like ones, our guides to detritus worms and aquarium snail eggs cover the other things people find in a tank.
Where do scuds come from?
Scuds are hitchhikers. They usually arrive as tiny individuals or eggs on new plants, on decor from another tank, or in with live or pond-collected food, then quietly multiply once they find a home.
Here is the reassuring part: finding scuds is not a sign that something is wrong. In fact, it usually means the opposite. Scuds thrive where there is biofilm, plant matter, and organic material to eat, so their presence often signals a mature, living tank with plenty of micro-life. They are not a symptom of a dirty tank, they are a symptom of a working one.
Are scuds good or bad? The honest answer
The truth is that scuds are both a cleanup crew and a live food at the same time, and whether that is good or bad comes down to your specific tank.
The good. Scuds eat detritus, leftover food, fish waste, and decaying plant matter, so they quietly help keep the tank clean. And because they breed constantly, they become a self-replenishing supply of high-protein live food. Most fish absolutely love hunting them, and a tank with a scud population gives your fish natural enrichment and great condition, for free.
The nuisance. Scuds only cause trouble in a few situations, and even then it is usually about competition and numbers, not aggression. They compete with shrimp for food, and in a dense population they will pick at freshly molted, soft, or weak shrimp and eat shrimp larvae and eggs. They can do the same to tiny fish fry. And in any overfed tank, all that extra food lets their population boom until they are everywhere.
Here is how it breaks down by tank type:
| Tank type | Scuds are | Why |
|---|---|---|
| Community fish tank | Good | Free cleanup and live food your fish will eat |
| Planted or natural tank | Mostly good | Helpful cleanup; only huge numbers nibble soft new growth |
| Shrimp-only tank | A nuisance | They compete for food and can pick at molting or weak shrimp and eat larvae |
| Fry or breeding tank | A nuisance | They compete with and can prey on tiny fry and eggs |
| Any overfed tank | A problem | Too much food lets their numbers explode |
Should you keep scuds or get rid of them?
The decision is simple once you know your tank. If you keep a community or planted tank with fish, keep the scuds and enjoy them. Your fish will crop the population naturally, and you get a free cleanup crew and live food out of the deal. If you keep a dedicated shrimp tank or you are raising fry, then you will want to control them, because there are no fish in there to eat them and they compete directly with the animals you care about.
How to keep and even culture scuds
If your fish eat them, scuds are one of the best free foods you can have. Many breeders go further and culture them on purpose.
A scud culture is easy. Set up a tub or spare tank with dechlorinated water, add plenty of leaf litter, moss, and a few hardy plants for them to hide and feed in, seed it with some scuds, and keep it gently filtered or aerated. Feed lightly with fish food, blanched vegetables, or more leaf litter, and they will breed steadily. Then just net or baste some out to feed to your tank. Nano fish like scarlet badis, galaxy rasboras, and pygmy corydoras take smaller scuds, while bigger fish happily eat the larger ones.
How to get rid of or control scuds
If you are a shrimp or fry keeper and the scuds are competing with your animals, here are the honest options, roughly easiest first:
- Add fish, if you can. The single easiest fix in a fish tank is a fish that eats scuds, which is almost all of them. This obviously does not work in a shrimp-only tank.
- Stop overfeeding. Scud numbers track the food supply, so feeding less shrinks a booming population over time.
- Bait and remove them. Put a boiled vegetable or a pellet on the substrate, wait for the scuds to gather on it at night, then lift or siphon it out. Repeat.
- Use a turkey baster. A baster sucks up individual scuds far better than a net, which mostly just catches your shrimp and snails too.
- Try a planaria-style trap. The little bait traps sold for planaria also catch scuds, though it is a slow way to thin a large population.
Two honest points. First, once scuds are established they are very hard to fully eradicate, so aim to control the population rather than wipe it out. Second, skip the harsh “cures” like dosing salt or fizzy water into a stocked tank. They stress or harm your fish and shrimp more than they reliably fix a scud problem.
Common scud myths
- “Scuds attack and kill my fish.” Overblown. Scuds are scavengers, not predators of healthy animals. They pick at things that are already dead, dying, or freshly molted and soft, not healthy adult fish.
- “Scuds destroy planted tanks.” Only in extreme numbers, and even then they mostly graze the softest new growth and decaying leaves rather than healthy plants.
- “Scuds mean my tank is dirty.” No. They mean your tank has enough biofilm and organic life to support them, which is usually a sign of a mature, healthy system.
FAQ
Are scuds harmful to fish?
No. Scuds are harmless to healthy fish and are actually a great natural food for them. They only cause problems in shrimp-only or fry tanks, where they compete with the animals you are keeping.
Do scuds eat shrimp?
Not healthy adult shrimp. In large numbers they will pick at freshly molted, soft, or weak shrimp and eat shrimp larvae and eggs, which is why they are a nuisance in a dedicated shrimp tank but a non-issue in a fish tank.
Where do scuds come from?
They hitchhike in on new plants, on decor from other tanks, or with live and pond foods, then multiply. Finding them is usually a sign of a healthy, biofilm-rich tank rather than a problem.
How do I get rid of scuds?
The easiest way is to keep fish that eat them. In a shrimp or fry tank, cut back on feeding, remove them with a baited trap or a turkey baster, and accept that controlling their numbers is more realistic than fully eradicating them.
Can fish eat scuds?
Yes, and most love to. Scuds are a nutritious, high-protein live food, and many people deliberately culture them to feed their fish.
Are scuds a sign of a dirty tank?
No. They are a sign that your tank has plenty of biofilm and organic matter to eat, which usually points to a mature, established, healthy tank.