Quick answer: Planaria are small free-living flatworms, usually 2 to 15 mm long, with a triangle-shaped head and two dark eyespots that make them look cross-eyed. They glide smoothly over the glass and substrate, and they’re white, gray or brown. In a fish-only tank they’re mostly a harmless nuisance, but in a shrimp tank they’re a real problem, because they eat baby shrimp, shrimp eggs, snail eggs and fish eggs, and can attack molting shrimp. The cause is almost always too much food. What most people get wrong is squishing or cutting them, since planaria regenerate from pieces. The fix is to cut feeding, trap them, and treat with a planaria medication if needed.
Planaria at a glance
| Detail | Planaria |
|---|---|
| What they are | Free-living flatworms (planarians), not parasites |
| Common aquarium types | Brown Dugesia and white planaria |
| Size | About 2 to 15 mm, sometimes up to 0.6 in or more |
| Color | White, cream, gray or brown |
| Head | Triangle or arrow shaped, with two dark eyespots |
| Movement | Smooth, slow gliding, like a tiny snail without a shell |
| Most active | At night and around feeding time |
| Diet | Meaty leftovers, tiny invertebrates, eggs, baby shrimp |
| Harmful to fish? | Not to adults. They can eat fish eggs and tiny fry |
| Harmful to shrimp? | Yes, to babies, eggs and molting shrimp |
| Harmful to people? | No |
| How they arrive | On plants, decor, live food or water from other tanks |
| Main cause of a boom | Overfeeding, especially protein foods |
Planaria vs detritus worms vs leeches vs nematodes: what do you have?
| Critter | Shape | Head | Movement | Harmful? |
|---|---|---|---|---|
| Planaria | Flat, wide body | Triangle head with two eyespots | Smooth gliding | Yes, to shrimp and eggs |
| Detritus worms | Thin, round, thread-like | Pointed, no eyespots | Wriggles and waves from the substrate | No |
| Nematodes | Very thin, round, clear or white | No visible head | Fast whip-like thrashing | Usually no |
| Leeches | Flattish, often brown | Sucker at each end | Inchworm-style stretching and contracting | Some species bite snails or fish |
| Rhabdocoel flatworms | Small, flat | Rounded head, no clear triangle | Fast gliding | No |
| Scuds | Tiny curled shrimp-like crustaceans | Antennae, legs | Fast darting, swims on side | Only in shrimp and fry tanks |
Here’s the honest part most planaria guides leave out. Almost every planted tank has a few planaria hiding in it, and you’ll only notice them when there’s enough leftover food for them to multiply. Before reaching for a treatment, check the head with a magnifier or your phone camera zoomed in. Plenty of people dose their tank for “planaria” that turn out to be harmless detritus worms or rhabdocoel flatworms. The triangle head and two eyespots are the giveaway.
Where do planaria come from?
- New plants and moss, the most common source
- Decor, rocks or wood from another tank
- Live or wild-caught foods and pond water
- Water from shops or other hobbyists’ tanks when you add new fish or shrimp
A few planaria can live unnoticed for months. Then a period of heavy feeding, or a fish that died and wasn’t found, gives them a feast, and suddenly they’re all over the glass.
Are planaria harmful to shrimp and fish?
Shrimp: yes, especially in breeding tanks. Planaria eat shrimplets and eggs, and they can wrap around and feed on shrimp that are molting or weak. A shrimp colony that stops growing even though females are carrying eggs is a classic sign. Our shrimp molting guide explains why freshly molted shrimp are so vulnerable.
Snails: planaria eat snail eggs and may attack tiny baby snails.
Fish: adult fish are safe. Fish eggs and very small fry are at risk.
People: aquarium planaria are harmless to humans.
How to get rid of planaria in an aquarium
Start with the steps that fix the cause, then use a treatment if the numbers stay high.
- Feed less. Feed only what your fish and shrimp finish in a few minutes, and cut back on protein foods. This is the most important step and works even without medication.
- Siphon the substrate. Remove leftover food, waste and anything dead. Check behind decor for a dead fish or snail.
- Trap them overnight. Put a small piece of raw shrimp, fish or a protein pellet in a planaria trap or a bottle with small holes, and leave it overnight. Planaria crawl in to feed, and you remove them in the morning. Repeat every night for a week or two.
- Don’t squish or cut them. Pieces can regrow into new worms. Remove them whole and throw them away.
- Treat if needed. If they keep coming back, use a planaria treatment (see below).
- Do a follow-up round. Eggs and survivors can hatch weeks later, so repeat trapping, or a second treatment if the product label says so.
Planaria treatments compared
| Treatment | How well it works | Shrimp | Snails | Notes |
|---|---|---|---|---|
| Fenbendazole (sold as dog dewormers like Panacur C or Safe-Guard) | Very effective | Usually safe at correct doses, some sensitive shrimp may be affected | Kills most snails | Also kills hydra. Dose carefully by tank volume |
| No Planaria (betel nut extract) | Very effective | Considered shrimp safe | Can harm snails, remove valued snails first | Made for shrimp tanks. Follow the label |
| Traps | Good for control | Safe | Safe | Rarely removes every worm on its own |
| Feeding less and cleaning | Slow but long lasting | Safe | Safe | Stops them coming back |
| Copper | Kills planaria | Kills shrimp | Kills snails | Don’t use in invert tanks |
Before and after treating:
- Move snails you want to keep, like nerite or mystery snails, to another tank first.
- Dose for the real water volume, not the tank’s label size. Our aquarium volume calculator helps.
- Add an air stone, since dying worms use oxygen and foul the water.
- Remove dead planaria and do a 30 to 50 percent water change a few days after treatment.
- Run activated carbon afterward to remove leftover medication before returning snails.
What eats planaria?
| Predator | How well | Shrimp safe? |
|---|---|---|
| Bettas | Sometimes | May eat baby shrimp |
| Gouramis, like the honey gourami | Sometimes | May eat baby shrimp |
| Pea puffers | Good | No |
| Scarlet badis and other micro predators | Sometimes | Eat baby shrimp |
| Small tetras and rasboras | Low, mostly tiny planaria | Usually safe with adult shrimp |
Fish help keep numbers down but rarely clear an outbreak, and most fish that eat planaria will also eat shrimplets. In a shrimp tank, feeding less, traps and a shrimp-safe treatment are the better tools.
How to prevent planaria in an aquarium
- Feed small amounts and remove leftovers within a few hours.
- Quarantine or dip new plants and moss. Rinse them well, or keep them in a bucket for a week or two and check for worms.
- Avoid adding shop water to your tank when buying fish or shrimp.
- Remove dead fish, snails and shrimp right away.
- Keep the substrate clean with weekly siphoning.
- Cycle new tanks properly before adding shrimp. See our how to cycle a fish tank guide.
Common planaria problems
Hundreds on the glass after feeding. Overfeeding, usually with protein. Cut feeding for a week and trap every night.
Shrimp colony not growing. Planaria may be eating the babies. Check the glass with a flashlight at night.
They came back after treatment. Eggs or survivors hatched. Do a second round and fix feeding.
Snails died after treating. Fenbendazole and some planaria treatments kill snails. Move snails out before dosing.
White worms that wriggle in the water column. Those are usually detritus worms, not planaria.
Tiny green or brown “tentacled” things on the glass. Probably hydra, another overfeeding pest. Fenbendazole treats both.
Worms in a brand new tank. They came on plants or decor. Trap them and quarantine new plants next time.
FAQ
What are planaria in an aquarium?
Small free-living flatworms with a triangle head and two eyespots. They glide over glass and substrate and feed on leftover food and tiny animals.
Are planaria harmful to fish?
Not to adult fish. They can eat fish eggs and very small fry.
Are planaria bad for shrimp?
Yes. They eat baby shrimp and eggs and can attack molting shrimp, so they’re a real problem in shrimp breeding tanks.
How do I get rid of planaria in my aquarium?
Feed less, siphon the substrate, trap them overnight with meaty bait, and use a planaria treatment like fenbendazole or No Planaria if they keep coming back.
How do I get rid of planaria naturally?
Cut feeding, clean the substrate and trap them every night. Without extra food, their numbers drop on their own.
What causes planaria in a fish tank?
They usually arrive on plants or decor, then multiply when there’s too much uneaten food.
What is the difference between planaria and detritus worms?
Planaria are flat with a triangle head and eyespots and glide smoothly. Detritus worms are thin and round, with no eyespots, and wriggle.
Can planaria live with snails?
Adult snails are usually fine, but planaria eat snail eggs. Many planaria treatments harm snails, so move them out before treating.
