Scuds in a Shrimp Tank: What They Are and How to Get Rid of Them Safely

You added a clump of moss two weeks ago, and now there's something in the tank that isn't a shrimp. It's small, curved like a comma, and it scoots along on its side in fast jerky bursts before vanishing into the substrate. That's a scud, and if you've spotted one there are more.

Scuds divide shrimp keepers more than almost any other hitchhiker. Some people feed them to their fish on purpose. Others tear a tank down to be rid of them. This guide covers what scuds are, how they got in, what they do and don't do to shrimp, and how to thin them out without reaching for a treatment that kills shrimp too.

Moss is the most common way scuds arrive; a clump this size can hide dozens

Quick Answer

Scuds are freshwater amphipods, small crustaceans in the genera Gammarus and Hyalella that run 3 to 20mm depending on species. They arrive on plants, in live food cultures, or in bag water, and they eat the same detritus, algae and biofilm your shrimp do. They don't hunt healthy adult shrimp, but a large population outcompetes shrimp for food, and several sources report scuds nibbling freshly molted shrimp and newly hatched shrimplets.

Get rid of them the way you'd get rid of planaria without chemicals: feed less, trap them overnight with a baited planaria trap, and net them out by hand. Skip copper, AlgaeFix, Excel overdoses and CO2 bombs. Scuds are crustaceans, so anything that reliably kills them kills shrimp.

What Scuds Actually Are

Scuds are amphipods, an order of crustaceans with marine and freshwater members. The two you'll meet in an aquarium are Gammarus and Hyalella, and the difference matters because they're different sizes.

Hyalella azteca is the common North American scud, and it's small: 3 to 8mm as an adult, usually white, green or brown. It's commonly sold as live food.

Gammarus species are bigger, typically 10 to 20mm, with males of Gammarus pulex reaching 21mm. These are the "freshwater shrimp" of European ponds and streams.

Both share the amphipod body plan: flattened side to side, no carapace, a curved back, and a habit of swimming on their side, which is where the name "side-swimmer" comes from. Amphipods have no larval stage. The female carries her eggs in a brood pouch under her body and releases fully formed miniatures, 20 to 30 at a time every few weeks in warm water. Adults live about a year and molt every few days. A generation time measured in weeks is how a handful of hitchhikers becomes hundreds by the end of the month.

One piece of biology is useful for Caridina keepers: Hyalella azteca doesn't survive below pH 6.0. Wikipedia states it, and a published lake survey found the species absent from every lake at pH 5.0 to 5.4 and present in 23 of 25 lakes above pH 6.0. That's a field result, not a treatment, but it may explain why buffered soil tanks running below 6.0 seem to see fewer Hyalella outbreaks. I found less hard data on Gammarus, so don't count on low pH to protect a tank from the larger species.

How Scuds Get Into a Shrimp Tank

Scuds walk in the same doors as every other hitchhiker, roughly in this order of likelihood:

1. •Live plants, especially moss. Every source names this first. Juvenile scuds are tiny and hide in moss, plant roots and the base of stem bunches.
2. •Live food cultures. Daphnia, scud and blackworm cultures from other hobbyists often carry Hyalella. If you culture live food, keep it well away from the shrimp tank.
3. •Bag water and livestock. Scuds ride in with fish, snails and shrimp from other tanks, sometimes clinging to a snail shell.
4. •Used substrate, filters, driftwood and rocks. Anything from a pond or another aquarium, particularly collected wood.

The only real prevention is quarantine. Two weeks in a bare container tells you what came in with a plant, and a plant dip before that catches most of the rest.

Do Scuds Eat Shrimp?

Here's where the sources disagree, so I'll lay out both sides.

The "harmless" side. Scuds are detritivores. They eat decaying plant matter, algae, biofilm, leftover food, and dead animals. AquaInfo's pest guide calls amphipods "harmless" outright. TankZen says they "generally coexist peacefully with cherry shrimp and Amano shrimp" and are "not predatory toward healthy adult fish or shrimp." Shrimpkeepers agrees that scuds "don't hunt or attack healthy shrimp" and describes the molting shrimp reports as disputed. Nobody I found claims scuds kill healthy adult shrimp.

The "not so harmless" side. Aquatic Arts, which sells scud cultures, warns on its own product page that "scuds will also sometimes eat newly molted dwarf shrimp and their fry." Fish Keeping Academy says they will attack shrimp "that have recently molted." Superior Shrimp and Aquatics reports that shrimplet survival "drops significantly" in tanks with an established scud population and that some scuds eat shrimp eggs. Planted Tank Masters goes furthest and says they're "dangerous and should not be kept" with shrimp.

Where everyone agrees: competition. Scuds and dwarf shrimp eat the same things, and scuds are faster, more aggressive foragers with a shorter breeding cycle. In a 10 gallon tank a scud population will eventually out-eat and out-breed the shrimp. Aquariadise and Planted Tank Masters both make this the central concern, and it's the one I'd take most seriously.

My read of all this: a few scuds in a well fed tank do nothing. A few hundred scuds in a tank where food is short will strip the biofilm shrimplets depend on, and a newly molted shrimp sitting still in a corner is a soft target for an animal that eats anything it can chew. The disagreement in the sources is mostly a disagreement about population size.

Scuds also nibble plants, with the same caveat. Most sources say they go for moss and soft plants once other food runs low; TankZen says they don't eat plants at all. Keepers who've lost a moss carpet to scuds would side with the majority.

Scuds vs Shrimplets vs Seed Shrimp: Telling Them Apart

New keepers regularly mistake scuds for baby shrimp, and vice versa. Hold a flashlight to the glass after lights out and check these points.

The single quickest tell is orientation. A shrimplet stands on its legs and swims upright; a scud lies on its side and moves like a flicked fingernail. Seed shrimp and copepods are covered in our seed shrimp guide, and if you're seeing thin wriggling threads instead, see detritus worms in shrimp tanks.

How to Get Rid of Scuds Without Hurting Shrimp

Every method below is safe to run with shrimp in the tank. None will get you to zero in a week; the goal is to knock the population down to a level the tank can carry, then keep it there.

◆Feed less

Scud populations track food supply. TankZen suggests cutting feeding by 25 to 30 percent for a week; Shrimpkeepers says to feed only what the shrimp clear in 1 to 2 hours. Pull uneaten wafers and vegetable slices before bed. This alone stops a population from growing. If you're unsure whether you've been overfeeding, a nitrate reading above 20ppm on an API Freshwater Master Test Kit usually answers the question.

◆Trap them overnight

A glass planaria trap is the tool most keepers settle on. Put a pinch of shrimp food, a sliver of blanched zucchini, or a piece of fish or shrimp pellet in the bulb end, lay it on the substrate in a dark corner at lights out, and lift it in the morning. Scuds crawl in for the bait and can't find the way back out. Empty it, rebait, repeat; expect a few nights before the catch drops.

The DIY version works nearly as well: cut the top third off a small plastic bottle, invert it into the base as a funnel, bait it, and weigh it down. Shrimp will wander into either trap too; just tip them back into the tank.

◆Bait and net

Drop a slice of blanched zucchini, carrot or cucumber into a fine mesh net and rest it on the substrate for a few hours or overnight. Scuds cluster on the vegetable; lift the net straight up and shake it out in a bucket. Planted Tank Masters and Aquariadise both describe this method, and it doubles as a rough census of how bad the problem is.

◆Vacuum the substrate

Gravel vacuum during water changes, concentrating on the corners and under hardscape where scuds hide by day. This removes juveniles and any adults that don't scatter fast enough.

◆Fish, with a big caveat

Bettas, gouramis, pea puffers, small loaches and most tetras eat scuds enthusiastically. The problem: every one of those eats shrimplets at least as enthusiastically, and Planted Tank Masters notes fish "probably can not fully eradicate" scuds anyway because the adults hide in substrate and moss. There's no fish that's both a reliable scud predator and shrimplet safe.

Two ways fish can still help: move infested plants and hardscape into a fish tank for a month and let the fish clean them, or add a small predator to the shrimp tank only while the colony isn't breeding and pull it before shrimplets appear.

◆The nuclear option

If scuds have overrun a tank and you'd rather start fresh, the only sure method is a full breakdown: shrimp out, substrate discarded, tank and hardscape scrubbed with hydrogen peroxide, filter media replaced. Aquariadise and Planted Tank Masters both list this, and both treat it as a last resort.

Treatments to Avoid in a Shrimp Tank

Scuds and shrimp are both crustaceans. That single fact rules out most of what the internet suggests.

• •Copper. Copper sulfate at 1.5 to 2mg/L kills scuds, and it kills every shrimp in the tank at a fraction of that. Never use a copper based medication with shrimp.
• •API AlgaeFix. The EPA label reads: "Do not use with freshwater crustaceans, including shrimp, crabs and lobsters." Some scud guides recommend it anyway. Don't.
• •Seachem Excel overdose. Ten to fifteen times the normal dose of glutaraldehyde does kill scuds, along with moss, plants, snails and shrimp.
• •The CO2 or soda water method. Pouring half a liter or more of seltzer into the tank suffocates scuds. Every source describing it says to remove fish, snails and shrimp first, which makes it a full teardown in disguise.
• •No Planaria and fenbendazole. These are the two "shrimp safe" pest treatments, and they're aimed at flatworms and hydra, not crustaceans. Genchem's product page for No Planaria lists planaria as the target; Gensou's treatment guide says the same for both products and never mentions scuds. A couple of scud guides list them as scud killers, but I couldn't find consistent keeper reports of either working on amphipods, and both are hard on snails at treatment doses. Don't dose a tank on the hope that it might.

Living With a Few Scuds

Not every scud is a crisis. In a tank with fish, scuds are free live food; Hyalella is sold by the culture for exactly that reason. In a shrimp-only tank that's fed sparingly, a small population sits at a few dozen and eats the mulm you'd otherwise vacuum out.

The tipping point is when you count scuds faster than shrimp on the glass at night, when moss starts thinning, or when berried females stop producing visible shrimplets. At that point run the traps, cut the food, and give it a month. If you're trying to grow out a batch of babies in the meantime, our shrimplet guide covers the extra cover and feeding that protect them.

Frequently Asked Questions

◆Are scuds harmful to shrimp?

Scuds don't attack healthy adult shrimp, and small numbers are harmless. Large populations outcompete shrimp for food, and several retailers and guides report scuds eating newly molted shrimp and shrimplets. Treat a few scuds as a curiosity and a booming population as a problem for breeding.

◆How do I tell scuds from baby shrimp?

Scuds lie on their side, have a curved comma-shaped body flattened side to side, and move in fast jerky scoots. Shrimplets look like perfect 2mm copies of an adult shrimp, stand upright on their legs, and swim forward. Seed shrimp are smaller still, oval, and dart in short straight lines.

◆What kills scuds but not shrimp?

Nothing you can pour in. Scuds and shrimp are both crustaceans, so copper, AlgaeFix and CO2 dosing kill both. The shrimp-safe approach is mechanical: cut feeding, run a baited planaria trap every night, net scuds off blanched vegetables, and vacuum the substrate. It takes weeks rather than days.

◆Where did scuds in my tank come from?

Almost always from live plants, and moss in particular, where tiny juveniles hide unseen. Live food cultures, bag water from other tanks, and used substrate or driftwood are the other common routes. A two week quarantine for new plants catches them before they reach the shrimp.

◆Do scuds eat plants?

Most sources say yes when food is short: moss, hornwort and other soft plants are the first to go, and a large colony can strip a moss carpet. A minority of guides say they leave plants alone. In a well fed tank with a small population, plant damage is unlikely.

The Bottom Line

Scuds are scavengers, not predators, but they breed faster than shrimp, eat the same food, and won't turn down a soft molting shrimp or a newborn. Keep the population small with light feeding and a nightly trap, and never reach for a crustacean killer to fix a crustacean problem.

If the small things in your tank turn out to be something else, our planaria and hydra guide covers the pests that do hunt shrimp.