Quick answer: Scuds are small freshwater amphipods, usually 2 to 10 mm long, that look like tiny curled shrimp and swim fast on their sides. They’re harmless to adult fish, people and healthy plants, and most fish love eating them. In a community tank, scuds are a free cleanup crew and live food. In a shrimp or fry tank, they’re a nuisance, because they compete for food and can pick at baby shrimp, eggs and fish fry. They usually arrive on new plants, and a population boom means there’s too much food. What most people get wrong is reaching for chemicals. Most treatments that kill scuds also kill shrimp and snails. Cutting food, trapping and scud-eating fish work far better.
Scuds at a glance
| Detail | Scuds |
|---|---|
| What they are | Freshwater amphipods (crustaceans), not true shrimp |
| Common genera | Gammarus (larger) and Hyalella (tiny, common on plants) |
| Size | About 2 to 10 mm, some Gammarus up to 20 mm |
| Color | Gray, brown, greenish or see-through |
| Shape | Curled, C-shaped, flattened from side to side |
| Movement | Darting, often swimming on their side |
| Diet | Detritus, leftover food, biofilm, dead plant matter |
| Harmful to fish? | No, but they may eat fish eggs and fry |
| Harmful to shrimp? | Can compete with and pick at baby or molting shrimp |
| Harmful to people? | No |
| How they arrive | On new plants, decor, or in live and pond-collected food |
| Main cause of a boom | Overfeeding |
Scuds vs copepods vs seed shrimp vs baby shrimp: what do you have?
| Critter | Size | Shape | Movement | Harmful? |
|---|---|---|---|---|
| Scuds (amphipods) | 2 to 10 mm | Curled, shrimp-like, flat sides | Fast darting, swims on side | Only in shrimp and fry tanks |
| Copepods | Under 2 mm | Teardrop body, often with egg sacs and long antennae | Jerky hops in open water | No |
| Seed shrimp (ostracods) | Under 2 mm | Round, like a tiny seed or bean | Smooth gliding | No |
| Baby shrimp | 2 to 5 mm | Straight, miniature adult shrimp | Walks and grazes, swims upright | No, they’re your shrimp |
| Detritus worms | Up to about 1 in | Thin white worm | Wriggles | No |
| Planaria | Up to 0.6 in | Flat worm with a triangle head | Glides smoothly | Yes, to shrimp |
The easiest test is the swim. Scuds dart quickly and often tip onto their side, while baby shrimp walk or swim upright and spend most of their time grazing. If you see thin white worms instead, our detritus worms guide covers those.
Here’s the honest part most scud guides leave out. Scuds aren’t a sign of a dirty tank. They’re a sign of a living one, with biofilm, leaf litter and leftovers to eat. A small, stable scud population in a fish tank is a good thing. Only two situations really call for control: a shrimp breeding tank, and a tank where you’re raising fry or eggs. Everywhere else, the “problem” is usually just too much food.
Where do scuds come from?
- New live plants and moss, the most common source
- Decor, rocks or wood from another tank
- Pond plants, pond water or wild-collected live food
- Water from shops or other hobbyists’ tanks
To prevent them, rinse new plants well, inspect moss closely, or quarantine plants in a bucket for a week or two before adding them.
Are scuds good or bad?
| Tank type | Verdict | Why |
|---|---|---|
| Community fish tank | Good | Free cleanup crew and live food. Fish keep numbers down |
| Planted tank with small fish | Mostly good | They clean detritus. Watch numbers if you have lots of moss |
| Shrimp breeding tank | Bad | Compete for food, may pick at baby and molting shrimp |
| Fry or egg tank | Bad | May eat eggs and very small fry |
| Snail breeding tank | Mixed | Compete for food and may eat snail eggs |
Are scuds harmful to shrimp?
To adult shrimp, rarely. Adult cherry, Amano and other shrimp are usually left alone.
To baby and molting shrimp, they can be. Scuds breed fast and compete for the same biofilm and food. In heavy numbers, they may pick at weak, molting or newborn shrimp. Many shrimp keepers see fewer babies surviving in tanks full of scuds.
If you breed shrimp, keep scud numbers low with the steps below. Our Amano shrimp guide covers shrimp tank care in more detail.
Do scuds eat plants?
Mostly no. They prefer dead and melting leaves, detritus and leftovers. When there are huge numbers and little food, they may nibble soft plants and mosses. That’s another sign of overpopulation, not a reason to panic.
Scud life cycle and breeding
Scuds breed easily in aquariums, which is why numbers can jump quickly.
- Pairing: males carry females under them for days, a behavior called amplexus, and mate right after she molts.
- Eggs: females carry about 15 to 50 eggs in a brood pouch under their body.
- Hatching: babies hatch as tiny, fully formed scuds in about 1 to 3 weeks. There’s no larval stage.
- Maturity: young can breed in roughly 1 to 2 months, faster in warm water.
- Lifespan: about 1 year, sometimes longer in cool water.
How to get rid of scuds or control them
Fully wiping out scuds in an established tank is very hard. Aim for control, starting with the safest steps.
- Feed less. Only what your fish or shrimp finish in a few minutes. This slows breeding more than anything else.
- Clean the substrate and filter. Siphon detritus and rinse sponges in old tank water. Less mulm means less scud food.
- Trap them at night. Scuds are most active after lights-out. Put a blanched carrot, zucchini or a pellet in a shallow dish or a mesh bag overnight, then lift it out in the morning. A planaria or shrimp trap with small holes also works.
- Use a turkey baster. Suck out groups of scuds from moss and corners.
- Add scud-eating fish to tanks without shrimp. See the table below.
- Remove and rinse moss in a bucket of tank water. Many scuds will shake out.
- For a shrimp tank with a bad infestation, move the shrimp to a separate tank, then strip and clean the old one thoroughly before restarting it.
What not to do. Avoid copper, fish dewormers, overdosing liquid carbon or fizzy water. They’re unreliable against scuds, and most will kill your shrimp and snails too.
What fish eat scuds?
| Fish | How well they eat scuds | Shrimp safe? |
|---|---|---|
| Bettas | Good hunters | Sometimes eat baby shrimp |
| Pea puffers | Excellent | No |
| Scarlet badis | Very good, they love live food | Eat baby shrimp |
| Loaches like zebra, yoyo and clown | Good | No |
| Tetras, rasboras and danios | Moderate, mostly small scuds | Usually safe with adult shrimp |
| Guppies and mollies | Moderate | Eat baby shrimp |
| Corydoras | Low to moderate | Yes |
Almost any fish that eats scuds will also eat baby shrimp, so in a shrimp breeding tank, manual control is usually the better choice.
Can you keep or culture scuds?
Yes. Scuds are an excellent, high-protein live food. Set up a tub or bucket with dechlorinated water, an air stone, leaf litter, moss and a little substrate. Feed a small pinch of fish food or blanched vegetables every few days. Harvest them with a net or turkey baster and feed them to your fish.
Common scud problems
Hundreds of scuds after adding plants. They came on the plants and the tank has spare food. Feed less and trap at night.
Fewer baby shrimp surviving. Scuds are competing with them. Reduce scuds with traps and lighter feeding.
Scuds in the filter. The media is full of detritus. Rinse it in old tank water.
They came back after cleaning. A few always survive in moss and substrate. Keep up lighter feeding and weekly trapping.
Tiny white worms with the scuds. Usually detritus worms, from the same extra food. Fix feeding and both decline.
Jelly blobs on the glass. Those are snail eggs, not scud eggs. See our aquarium snail eggs guide.
FAQ
What are scuds in an aquarium?
Small freshwater crustaceans called amphipods, usually 2 to 10 mm long. They look like tiny curled shrimp and swim fast on their sides.
Are scuds harmful to fish?
No. Adult fish are safe, and most fish eat scuds. They may eat fish eggs and very small fry.
Are scuds bad for shrimp?
They can be in shrimp breeding tanks, because they compete for food and may pick at baby and molting shrimp. Adult shrimp are usually fine.
What is the difference between scuds and copepods?
Scuds are larger, about 2 to 10 mm, curled and swim on their sides. Copepods are under 2 mm, teardrop shaped and hop jerkily through the water.
How do I get rid of scuds?
Feed less, siphon the substrate, trap them overnight with blanched vegetables, and add scud-eating fish in tanks without shrimp.
What fish eat scuds?
Bettas, pea puffers, scarlet badis, loaches and most small tetras and rasboras will eat scuds.
Are scuds good for aquariums?
In most fish tanks, yes. They clean up leftovers and are great live food. Control them only in shrimp and fry tanks.
