Planaria and hydra in a shrimp tank: what they are and what to do

Published October 16, 2026 · Updated October 16, 2026 · 5 min read

A red cherry shrimp climbing through a tangle of green aquarium moss — Photo: Albertomeg, Wikimedia Commons, CC0 (cropped)

Quick answer

Planaria are small flatworms with a triangle-shaped head. Hydra are tiny white or green stalks with tentacles. Both thrive when there is too much food in the tank. Adult shrimp are rarely in danger, but hydra can catch baby shrimp, and planaria can attack shrimp while they molt. Start by feeding less and removing leftovers. Chemical treatments work, but they can kill snails and cause an ammonia spike.

How to tell them apart

Not every tiny white creature in your tank is a problem. Three are common, and only two of them can hurt shrimp.

Feature	Planaria	Hydra	Detritus worms
Looks like	A flat worm with a triangle-shaped head and two eye spots	A thin stalk with tentacles on top, like a tiny upside-down jellyfish	Very thin, white, thread-like worms
Size	Up to about 2.5 cm (1 inch) when stretched	From a few millimeters up to about 3 cm	About 3–13 mm (0.1–0.5 inch)
Movement	Glides slowly over glass and decor	Stays stuck to one spot most of the time	Fast, wriggling, snake-like
Harmful to shrimp?	Yes, to weak, small or molting shrimp	Yes, mainly to baby shrimp	No

Are they dangerous to shrimp?

Hydra sting small animals with their tentacles and pull them in. Adult shrimp are usually fine. Baby shrimp are in real danger, especially in their first month.

Planaria usually go after weak, small or dying animals. Shrimp that have just molted (shed their old shell) are an easy target, because their new shell is still soft.

Detritus worms are harmless. They eat waste, not shrimp. A sudden boom of them is a sign of too much food or waste in the tank.

Why they appear

All three thrive on leftover food. Overfeeding, especially with powder food, feeds them directly. It also feeds the tiny water creatures that hydra hunt.

They can also arrive as hitchhikers on new plants, wood or rocks. Keeping new plants in a separate bucket or tank for a while lets you spot them before they reach your shrimp.

Step 1: feed less

This is the first step for all three, and often the only one you need.

Feed smaller amounts, and remove leftovers after 2 to 3 hours. Our feeding guide explains how much to give.
Use a small feeding dish, so food doesn't spread through the moss and gravel.
Cut back on powder food for a while.
Keep up your normal small water changes and clean lightly.

For detritus worms and hydra, this alone often keeps the numbers under control. Planaria may need traps as well.

Step 2: remove them by hand or with traps

Hydra: suck them off the glass with a piece of airline tubing or a syringe. Don't squash them, because they can grow back from pieces. Repeat for a few days.

Planaria: use a planaria trap. This is a small container with tiny holes and a bit of food inside. Leave it in the tank for a few hours, then empty it. Repeat until it stays empty. Don't cut planaria up either, because each piece can grow into a new worm.

Pest	First steps	If that is not enough
Detritus worms	Feed less, clean lightly	Nothing more is needed
Hydra	Feed less, suck them out over a few days	A treatment, after removing snails
Planaria	Feed less, use a trap for 2 to 12 hours at a time	A treatment, after removing snails

Step 3: treatments, with care

If feeding less and traps are not enough, there are treatments for planaria and hydra. The common ones are based on fenbendazole, a dewormer, or on betel nut extract, sold as No Planaria. Before you use any of them, know the risks:

They often kill snails. Move snails out first, or expect to lose them. Some keepers report that snails kept dying for months after a treatment.
Dying worms cause an ammonia spike. Rotting worms release ammonia, which can kill more shrimp than the worms did. Remove dead worms, test ammonia and do water changes over the following days.
Dose for the real amount of water. Gravel, rocks and wood take up space, so the tank holds less water than its size suggests. Dosing for the full size can overdose your shrimp.
Follow the product's instructions. Doses shared online vary a lot, and some products were not made for aquariums.
Eggs can hatch later. A second round may be needed after 2 to 3 weeks.

Don't treat detritus worms with chemicals. They are harmless, and killing a large number at once can cause the same ammonia spike.

If shrimp are dying and you are not sure why, start with our guide to why shrimp die.

FAQ

Are planaria dangerous to shrimp?

They can be. Planaria usually attack weak, small or dying animals, and shrimp that have just molted are easy targets because their new shell is soft. Healthy adult shrimp are rarely the first victims.

Are hydra dangerous to shrimp?

Mostly to babies. Hydra sting and catch tiny animals with their tentacles, so baby shrimp in their first weeks are at real risk. Adult shrimp are usually too big to be caught.

Is fenbendazole safe for shrimp?

Many keepers use it against planaria and hydra in shrimp tanks, but the dose matters a lot. It often kills snails, and the dying worms can cause an ammonia spike. Remove snails first, dose for the real water volume and do water changes afterwards.

What are the tiny white worms in my shrimp tank?

Thin, fast, wriggling white worms are usually detritus worms, which are harmless. Flat worms with a triangle-shaped head that glide slowly over the glass are planaria. Detritus worms are a sign of too much food or waste, so feed less and clean lightly.

Sources

Shrimp Science: Hydra in Shrimp Tanks: How To Get Rid of Them
Shrimp Science: Why Are My Freshwater Shrimp Dying?
Shrimp and Snail Breeder: Hydra in a Shrimp Tank: Treatment
Shrimp and Snail Breeder: Planaria and Shrimps: How to Remove Them
Shrimp and Snail Breeder: Detritus Worms in Aquarium: Harmless or Dangerous?
Shrimp and Snail Breeder: Shrimp Feeding Guide: How Much, How Often, and When to Stop
The Shrimp Farm: Cherry Shrimp Feeding

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