What is This Thing in My Tank?

What is This Thing in My Tank

This article is written with the main aim to help identify some of the common bugs and worms in our shrimp/fish tanks with pictures and illustrations. It can be especially useful for beginner aquarists who may be freaked out by unfamiliar creatures appearing in their tanks.

It is worth noting that there is a huge variety of aquatic organisms (potentially, – thousands!) that can inhabit our tanks, ranging from simple nematodes and hydras to leeches and water beetles.

Obviously, it is absolutely not possible to cover all of them. Therefore, I will focus on describing and showcasing the most common ones – the notorious ‘troublemakers’ of the aquarium world – and categorize them based on their impact on shrimps and fish.


• Hydra oligactis (Brown hydras)Hydras are small freshwater organisms belonging to the genus Hydra (class Hydrozoa). These creatures resemble skinless umbrellas or tiny corals. They can grow up to 0.4 inches (1 cm) in size and come in colors ranging from translucent white to green.

Hydras are predatory organisms. They are classic ambush predators that sit and wait for prey to come close enough to strike. 

They use their tentacles, which have stinging cells, to paralyze and catch their prey. Although they primarily feed on small aquatic invertebrates such as water fleas, daphniacyclops, and copepods, hydras may still harm small shrimp and fry.

Hydra Shrimp/Shrimplets Fish/Fry
Threat level Medium Low

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Freshwater limpets

Freshwater Limpets in Freshwater Tank Stay or GoFreshwater limpets, also known as Acroluxus, are a type of mollusk belonging to the class Gastropoda.

These snails generally do not grow larger than 0.3 inches (8 mm) in length, and 0.15 inches (or 4 mm) in height and width. Their shell is thin-walled, elongated, cap-shaped, and without whorls. The color usually ranges from translucent and yellowish-grey to brown.

They are scavengers and primarily feed on algae and other organic matter. Freshwater limpets are absolutely harmless mollusks. They simply are not able to harm anybody in the tank.

Freshwater limpets Shrimp/Shrimplets Fish/Fry
Threat level No No

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Detritus Worms in Freshwater Tank - Differences between Detritus worms, and Planaria

Planaria are a type of non-parasitic, free-living flatworm. They are flat, elongated worms that usually do not grow more than 0.8 inches (2 cm) in length.

Planaria can be easily distinguished from other unwelcome worms in our tanks by a triangular head with a pair of light-sensitive eyespots.

These flatworms are carnivorous and feed on decaying organic matter, algae, and other small organisms (such as protozoans, snails, and other worms). They use their muscular mouths to suck up their prey and digest it in their central digestive cavity.

Usually, planaria do not attack shrimp and fish fry because they are too fast for them. If such an incident occurs, it is more likely that the affected individuals were weak or dying. However, molting shrimp can be an easy target for them.

Planaria Shrimp/Shrimplets Fish/Fry
Threat level Low-Medium Low

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Dragonflies and Damselflies Nymphs  

Dragonfly nymph bigDragonflies and damselflies have aquatic nymphs that undergo incomplete metamorphosis before transforming into adults.

The main differences between dragonfly and damselfly nymphs lie in their body shape and gill placement. Dragonfly nymphs have wider abdomens, stout bodies, and internal gills, while damselfly nymphs are longer and slenderer, with gills extending from the hind end.

On average, their size range from 0.4 to 2 inches (1.5 – 5 cm.) in length. However, some larger species can reach lengths of up to 7.5 centimeters (3 inches) or more.

Dragonflies and damselflies nymphs are fierce and voracious predators that will eat any animal they can catch, including shrimp and fish of their size.

Dragonflies/damselflies Shrimp/Shrimplets Fish/Fry
Threat level High Medium-high

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Diving Beetles  

Profile of Diving Beetles- Monsters in Shrimp and Fish TanksDiving beetles are the most diverse beetles in the aquatic environment. They can be found in virtually any freshwater habitat around the world.

They have specialized adaptations for flying, swimming, and diving, including flattened bodies, smooth streamlined shapes, fringed hind legs that act as oars, and wings. Depending on the species, their size ranges from 0.4 up to 1.5 inches (1 – 4 cm) in length.

Profile of Diving Beetles- Monsters in Shrimp and Fish Tanks - Diving Beetles LarvaeDiving beetles and their larvae are carnivorous predators. They are often referred to as “Water tigers” due to their extremely aggressive feeding behavior. Additionally, they are active hunters.

These hunting machines are so strong that may catch even small amphibians (such as newts, frogs, etc.). Keeping them in shrimp or fish tanks is absolutely not recommended.

Diving beetles Shrimp/Shrimplets Fish/Fry
Threat level Very high High

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How to Culture Daphnia in Aquariums. Female with eggsDaphnia, commonly known as ‘Water fleas’, are small planktonic crustaceans which typically 0.04 – 0.2 inches (1 – 5 mm) in length. The name “Water fleas” stems from the jump-like behavior they exhibit while swimming. 

They have a relatively large head with one simple compound eye and a body encased in a bivalve-like shell.

Due to their tiny size and fast reproduction rate, Daphnia are often used as live food in the aquarium hobby and in scientific research as test organisms for studying water quality and ecological interactions.

Daphnia Shrimp/Shrimplets Fish/Fry
Threat level No No (Live food)

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Cyclops Profile and Culture GuideCyclops, also known as freshwater copepods, are small crustaceans belonging to the order Cyclopoida which contains over 1500 species. They are known for their single eye, which gives them their name “Cyclops.”

These tiny (0.02 to 0.08 inches (0.5 to 2 mm) in length), free-swimming organisms are found in various freshwater environments where they serve as prey for many small fish and other aquatic organisms.

Cyclops are fast horizontal swimmers. In the water, they move in sudden leaps, with a burst speed of up to 75 mm/second!

Cyclops Shrimp/Shrimplets Fish/Fry
Threat level No No (Live food)

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What is This Thing in My Tank - RotiferRotifer are a group of microscopic, multicellular animals belonging to the phylum Rotifera.

They are characterized by their ciliated crown, called the corona, which plays a locomotion role by generating a rotating current of water around them. This motion allows them to move through the water and capture food particles.

In terms of size, rotifers are quite small. Most species range in size from as small as 0.004 to 0.08 inches (0.1 – 2 mm) in length.

In aquarium hobby, rotifers are commonly used as live food for fish fry.

Rotifer Shrimp/Shrimplets Fish/Fry
Threat level No No (Live food)

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Seed Shrimp Profile Ostracods in Shrimp and Fish TankOstracods are small crustaceans thet are also known as “Seed shrimp” or “Oyster shrimps.”

These tiny creatures are typically oval-shaped and enclosed in a bivalve-like shell, which is composed of two hinged valves made of chitin. Basically, they resemble a miniature clam or mussel. They are generally quite small, ranging about 0.04-0.08 inches (1 – 2 mm) in length.

Ostracods are scavengers and will feed on leftovers, algaebiofilm, detritus, and organic debris in the substrate and water column.

In the aquariums, they will not harm anybody.

Ostracods Shrimp/Shrimplets Fish/Fry
Threat level No No (Live food)

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Detritus Worms

Detritus Worms in Freshwater Tank in the substrateDetritus worms are aquatic worms that belong to a family of oligochaete worms called Naididae. By the way, there are over 800 species of these worms in this family.

Generally, they look like thin, white, tiny pieces of strings or threads. These worms are considerably small, with the size ranging from a millimeter to a few centimeters long (up to 1 inch but usually from 0.1 to 0.5 inches).

In aquariums they can be seen lying on the substrate, swimming freely in the water column, or latching onto the tank walls.

These worms are completely harmless as they only feed on tiny particles of organic detritus on the bottom of the tank. Thus, Detritus worms are important decomposers and contribute to the overall health and balance of the tank by helping to maintain its water quality.

Detritus worms Shrimp/Shrimplets Fish/Fry
Threat level No No (Live food)

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Mosquito larvae 

Mosquito Larvae Profile and How to Culture as Live FoodMosquito larvae, also known as “wrigglers,” are the second stage in the life cycle of mosquitoes.

They have a distinct head with two antennae, and their segmented body is slender and elongated in shape. Mosquito larvae vary in size depending on their species and developmental stage. However, on average, after hatching, they are usually less than 0.04 inches (1 mm) in length. However, at the 4th instar larvae can reach lengths of up to 0.4 inches (1 cm).

Mosquito larvae typically hang just below the water surface, breathing air through tubes located at the end of their abdomen.

Most species of mosquito larvae are filter feeders and primarily consume microscopic organic particles, algae, and microorganisms. 

In aquariums, Mosquito larvae play a role of live food.

Mosquito larvae Shrimp/Shrimplets Fish/Fry
Threat level No No (Live food)

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Water Louse

What is This Thing in My Tank - Water LouseAsellus aquaticus, commonly known as the Water louse or Aquatic sowbug, is a small crustacean that belongs to the family Asellidae.

Water louse is quite easy to identify by its two pairs of antennae. One pair is short, while the other is almost as long as the body. They have compound eyes, and their body is segmented into 8 segments and covered with a thin layer of chitin. They are typically brown or gray in color.

These aquatic creatures typically range in size from 0.24 to 0.47 inches (6 – 12 mm) in length. However, some individuals may grow slightly larger.

 They are omnivorous scavengers, feeding on detritus, algae, and other organic matter, playing an important role in the nutrient cycling of aquatic ecosystems.

They are not considered dangerous to shrimp and fish.

Water louse Shrimp/Shrimplets Fish/Fry
Threat level No No (Live food)

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Scuds In Shrimp And Fish TankScuds are small crustaceans that belong to the family Gammaridae.

They have a laterally compressed body with a distinctive curved shape, and they are adapted for swimming and crawling on aquatic vegetation and surfaces.

Generally, scud species can reach lengths of 0.4–0.8 inches (1–2 cm). They can be any of the following hues: white, grey, yellow, brown, reddish-black, tan, etc.

They are detritivores and primarily feed on decaying organic matter and algae. In shrimp tanks, they may compete with shrimp for food. In fish tanks, they can be a part of the fish food chain. The only problem is that in rearing tanks they may eat fish eggs.

Scuds Shrimp/Shrimplets Fish/Fry
Threat level No (food competition) No (Live food)

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Water mites

What is This Thing in My Tank - Water mitesWater mites are a type of arachnid that belongs to the superfamily Hydrachnoidea.

Water mites have a rounded or elongated body, often brightly colored, with 2 or 4 eyes and hook-like chelicerae. Their pedipalps are equipped with hooks or bristles on the last segment.

These are very tiny creatures, measuring 0.08-0.12 inches (2 – 3 mm) in size.       

Depending on the species, these small aquatic creatures are parasitic or predatory in nature. They attach themselves to the host and use their specialized mouthparts to pierce the host’s exoskeleton and consume their fluids.

Generally, they parasitize on aquatic insects and crustaceans, for example, Daphnia and Cyclops. They are safe for fish.

Scuds Shrimp/Shrimplets Fish/Fry
Threat level No (food competition) No (Live food)

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What is This Thing in My Tank - BryozoansBryozoans are small aquatic invertebrates that belong to the phylum Bryozoa.

They are colonial organisms, meaning they live together in colonies composed of many individual zooids. The color of bryozoans can vary, with some species being translucent or white, while others may be brown, yellow, or green.

Bryozoans are filter-feeding animals. They have a specialized feeding structure that helps them capture and consume plankton and other small particles from the water.

Unlike hydras, they are completely safe.

Bryozoans Shrimp/Shrimplets Fish/Fry
Threat level No No


What is This Thing in My Tank - PeritrichaPeritricha are a group of ciliate protozoans that belong to the class Peritrichia.

These protozoans are generally characterized by a cylindrical or vase-shaped body covered in cilia, which they use for movement and feeding. They are often found attached to surfaces in aquatic environments and are known for their diverse and fascinating shapes.

Peritricha are very small and primarily feed on bacteria, algae, and fungi.

In aquariums, they often disappear once the water balance stabilizes and the suspended particles diminish (due to a scarcity of food).

Peritricha Shrimp/Shrimplets Fish/Fry
Threat level No No


What is This Thing in My Tank - StentorStentor is a genus of large, trumpet-shaped, single-celled organisms, belonging to the ciliate group.

These organisms are among the largest known unicellular organisms and are commonly found in freshwater environments. They are typically around 0.04 inches (1 mm) in length when fully extended.

Stentor has a distinctive trumpet or bell-shaped body which is covered in numerous hair-like structures called cilia, which it uses for both locomotion and feeding.

Stentor are filter feeders, they use their cilia to create water currents that bring in food particles, such as bacteria and other small organisms, into their oral disc.

They are a natural part of freshwater ecosystems, and do not pose any danger to plants, fish, or shrimp.

Stentor Shrimp/Shrimplets Fish/Fry
Threat level No No


What is This Thing in My Tank - LeechesFreshwater leeches are a type of annelid worm that belong to the class Hirudinea.

They have elongated, worm-like bodies with a flattened shape. They can vary in color, ranging from dark brown to green, and some may have distinctive patterns or markings. At each end of their body, they have a sucker that helps them attach to surfaces or host organisms

Depending on the species, they can measure anywhere from 0.4 to 2 inches (1 – 5 centimeters) in length.

These leeches are typically parasitic or predatory. Some species are bloodsuckers, attaching to and feeding on the blood of fish, and amphibians, while others are predators that feed on small invertebrates (such as shrimp).

Leeches Shrimp/Shrimplets Fish/Fry
Threat level High High

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What is This Thing in My Tank - Snail leechesGlossiphonia sp. and Hetobdella sp. (also known as Snail-leech, Flattened leech, or River leech) is a species of freshwater leech.

They have flattened and segmented bodies, which are covered by a thin layer of chitin. The color often ranges from light yellowish or greenish to brown, becoming darker with age. They have three pairs of papillae rows on the body, with the medially located row being the most developed.

Snail-leeches can be easily identified by their droplet-shaped build. Due to their semitransparent body, it is often possible to see eggs and the digestive tract in the middle of their body. These snails typically grow between 0.4 – 1 inches (1 – 2.5 cm) in length.

If you detach the leech from the surface, it falls to the bottom and curls up into a ring.

As the name suggests, they primarily feed on water snails.

Snail-leeches Shrimp/Shrimplets Fish/Fry
Threat level No No


What is This Thing in My Tank - SpringtailsPoduridae is a family of tiny, primitive insects belonging to the order Collembola. They are also known as “springtails” due to their ability to catapult themselves into the air using a specialized structure called a furcula.

Springtails have a cylindrical body and are usually between 0.1 – 0.2 inches (2 – 5 mm) in length. They have 6 legs, a forked appendage called a furcula at the tip of their abdomen, and a pair of antennae on their head.

They primarily feed on decaying organic matter, fungi, algae, and bacteria. In aquariums, these small creatures do not pose any threat to larger aquatic animals. On the contrary, some fish species may even consume springtails as part of their natural diet.

Springtails Shrimp/Shrimplets Fish/Fry
Threat level No No (Live food)

Water Scorpions

What is This Thing in My Tank - Water scorpionsWater scorpions (Nepa sp.) are a type of aquatic insect belonging to the family Nepidae.

They have oval-shaped bodies and raptorial front legs that help them catch their prey. On average, their size can range from approximately 1.5 to 2 inches (3.5 – 5 cm).

Water scorpions are predators. Their front legs are adapted for grasping and capturing prey such as small aquatic insects, tadpoles, and even small fish.

Water scorpions Shrimp/Shrimplets Fish/Fry
Threat level High High

How to Avoid Unexpected Guests in the Tanks?

If you are tired of dealing with unexpected guests in your tanks, there are some proactive steps you can take to prevent this from happening.

  1. Quarantine new additions: Always quarantine new plants, and animals before adding them to your main tank. This helps prevent the introduction of potential pests or diseases.
  2. Disinfect decorations. By doing so, you can significantly reduce the risk of introducing harmful organisms and potential diseases.
  3. Regular cleaning: Regularly clean and inspect your tank and its contents. Remove any debris, uneaten food, or dead plant matter.
  4. Inspect live foods: If you use live foods, (such as daphnia, artemia, tubifex worms, etc.), inspect them carefully before feeding them to your fish to avoid introducing parasites.
  5. Avoid overfeeding: It will help you to prevent excess food from decaying and attracting unwanted organisms.

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In Conclusion

I bet that every aquarist has encountered the situation at least once in their life when some unexpected guests are found in their beloved aquarium.

Of course, some of these guests can be extremely dangerous and should be removed ASAP. Nonetheless, it is also important to remember that many of them occupy their own ecological niche and often play an integral role in the ecosystem.

2 thoughts on “What is This Thing in My Tank?

  1. Hello.
    We found out we have snail leeches in one of our shrimp tanks. We have taken out every surviving ramshorn snail to deplete their food source, and have been taking out every snail leech we see. The shrimp are still in the tank though since they don’t seem to bother with them; plus they are way too fast.

    We have been reading about snail leeches and have found out that they lay their eggs in pouches that they carry on them. When the leeches hatch they remain with their parents adhered to their body; this because their proboscis are too weak to penetrate a snail’s skin, so they have to rely on their parents to feed them untill they are old enough to hunt by themselves. Can’t seem to find more information, and based on what we have read they are extremely difficult to remove. We really hope you have had some experience with this little pests and you could help us out. Here are our questions:

    1. Given that juvenile leeches can’t eat by themselves, are eggs that fall off when you take them out something to worry about? acording to this they should hatch and then starve… right?
    2. This would also mean that to get rid of leeches you should just worry for the adults… true?
    3. If this is not viable, is there a way to get rid of them without tearing down a stable and mature tank?

    our shrimptank is only 3g, so manual removal could be a thing if this is the case. We use an undergravel filter.
    We thank you in advance for your time.

    1. Hi Javier,
      There are many species of leeches, and their reproductive methods, as well as the care of their offspring, can vary greatly. Some species attach cocoons to surfaces and do not play a role in the young leeches’ lives after that. Meanwhile, others (such as Glossiphonia) actually carry their young until they become self-sufficient.
      I only know of two ways to deal with them, which are the use of aquarium salt or alcohol-based products. You can check out the topic about Rabbit snails where I described this, as these snails are most commonly affected by this issue.
      Best regards,

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