Aquarium Pest Identification: Worms, Bugs and Hitchhikers

Table of Contents

• How Pests Arrive in Your Aquarium
• Pest Threat Level Summary Table
• Detritus Worms
• Planaria
• Hydra
• Copepods and Cyclops
• Ostracods (Seed Shrimp)
• Leeches
• Dragonfly and Damselfly Nymphs
• Scuds (Amphipods)
• Frequently Asked Questions

You peer into your aquarium one morning and spot something you did not put there. A tiny worm wriggling on the glass. A translucent, tentacled creature attached to a plant leaf. A fast-moving bug darting through the substrate. The immediate reaction is alarm, but before reaching for chemicals, you need to know what you are dealing with. Most aquarium hitchhikers are harmless or even beneficial. A few are genuinely dangerous and require prompt action.

This guide covers the most common uninvited guests found in freshwater aquariums in Singapore. For each, we explain how to identify it, whether it poses a threat and what to do about it. Over twenty years at our shop at 5 Everton Park, we have encountered all of these organisms and helped customers distinguish the harmless from the harmful.

How Pests Arrive in Your Aquarium

Almost every pest enters via the same routes:

• Live plants — the most common vector. Eggs, larvae and adult organisms cling to plant leaves, roots and stems. Even tissue-cultured plants can occasionally harbour microscopic hitchhikers.
• Driftwood and hardscape — especially if sourced from natural waterways.
• New fish and shrimp — the transport water and bags can carry organisms.
• Substrate — particularly second-hand or naturally sourced gravel and sand.
• Equipment — nets, siphons and buckets used across multiple tanks can transfer pests.

Quarantining new plants and livestock for one to two weeks in a separate container reduces the risk significantly. A simple dip in dilute potassium permanganate or alum solution before adding plants to the main tank kills most visible hitchhikers.

Pest Threat Level Summary Table

Detritus Worms

How to identify

Detritus worms (Naididae family) are thin, white or translucent, hair-like worms typically 1 to 2 centimetres long. They wriggle freely in the water column or crawl on the glass, substrate and filter media. They are most visible at night when they emerge from the substrate.

Threat level: Harmless

Detritus worms are decomposers. They feed on organic waste — uneaten food, fish waste, decaying plant matter. Their presence indicates a healthy, functioning ecosystem. They are part of the natural food web and many fish species eat them eagerly.

What to do

Nothing. If you see a sudden population explosion of detritus worms, it usually means you are overfeeding or the substrate has accumulated excess organic matter. Reduce feeding, vacuum the substrate and the population will decline naturally. Fish like bettas, tetras and corydoras will eat them.

Planaria

How to identify

Planaria are flatworms, typically 5 to 15 millimetres long, with a distinctive arrow-shaped or triangular head. They glide smoothly across surfaces — unlike detritus worms, which wriggle erratically. If you look closely (a magnifying glass helps), you can often see two eye spots on the head, giving them a cross-eyed appearance. They are usually white, cream or brown.

The key distinguishing feature: planaria move smoothly and deliberately. Detritus worms wriggle. If it slides rather than squirms, it is likely planaria.

Threat level: Dangerous to shrimp

Planaria are predatory. They hunt shrimplets, attack moulting adult shrimp (when the shell is soft) and can kill small snails. In a shrimp-only tank, a planaria infestation can devastate the colony. In a fish-only tank, they are mostly harmless — most fish are too large and active to be threatened.

What to do

• Fenbendazole — the most effective treatment. Available as dog dewormer (Panacur or generic fenbendazole). Dose at 0.1 gram per 40 litres. Kills planaria within 24 to 48 hours. Safe for fish and shrimp but lethal to snails. Remove snails before treatment if you wish to keep them.
• No-Planaria — a commercial product containing betel nut extract. Effective and widely available at aquarium shops in Singapore. Follow the dosing instructions on the package. Also kills snails.
• Planaria trap — a glass or plastic trap baited with raw meat attracts planaria overnight. Effective for reducing numbers but rarely eliminates the population entirely. Best used as a supplement to chemical treatment.

Hydra

How to identify

Hydra are tiny polyps (5 to 15 millimetres) that attach to glass, plants, driftwood or any hard surface. They have a tubular body with a ring of tentacles at the top, resembling a miniature sea anemone. When disturbed, they contract into a small blob. Colours range from translucent white (common hydra) to green (hydra with symbiotic algae).

Threat level: Dangerous to fry and shrimplets

Hydra are cnidarians — relatives of jellyfish and corals. Their tentacles contain stinging cells (nematocysts) that paralyse tiny prey. They capture and consume copepods, daphnia, newborn fish fry and shrimplets. Adult fish and adult shrimp are too large to be affected. In a shrimp breeding tank, hydra are a serious problem.

What to do

• Fenbendazole or No-Planaria — the same treatments used for planaria also kill hydra effectively. Same dosing, same precautions regarding snails.
• Manual removal — scrape hydra off the glass with a razor blade or old credit card. Not practical for large infestations but works for isolated clusters.
• Reduce feeding — hydra populations boom when there is abundant microscopic food (baby brine shrimp, crushed flake food, infusoria). Cutting back on fine foods reduces their food supply.
• Biological control — some gouramis and paradise fish eat hydra, but these species also eat shrimp, making this solution impractical in shrimp tanks.

Copepods and Cyclops

How to identify

Copepods are tiny (1 to 2 millimetres) crustaceans that dart rapidly through the water in jerky, stop-start movements. They are teardrop-shaped, often carrying visible egg sacs. Cyclops is a genus of copepod with a distinctive single eye spot. Under magnification, they look like miniature translucent shrimp.

Threat level: Harmless

Copepods are free live food for your fish. They feed on microalgae, bacteria and detritus. Their presence indicates a healthy, well-established aquarium with a functioning microfauna. Many hobbyists actively cultivate them.

What to do

Enjoy them. Your fish will eat most of them. In heavily planted tanks without fish, copepod populations can become very large, which simply means the tank has abundant organic matter. They are completely harmless to fish, shrimp and plants.

Ostracods (Seed Shrimp)

How to identify

Ostracods are tiny (1 to 2 millimetres) crustaceans enclosed in a bivalve (two-part) shell, making them look like round seeds bouncing through the water. They move in quick, jerky bursts, similar to copepods but rounder in shape. Under magnification, you can see tiny appendages protruding from the shell.

Threat level: Harmless

Ostracods feed on detritus, algae and bacteria. They are a natural part of the aquarium microfauna and serve as live food for fish. They are completely harmless to shrimp, plants and fish.

What to do

Leave them. Like copepods, their population is self-regulating based on food availability. Fish eat them readily. They are a sign of a biologically mature aquarium.

Leeches

How to identify

Aquarium leeches are segmented worms that move with a distinctive inchworm motion — attaching the front end, pulling the body forward, then attaching the rear end. They are flattened, often dark brown or black, and range from 5 to 30 millimetres. Unlike detritus worms, their movement is deliberate and directional.

Threat level: Parasitic (but rare)

True leeches are blood-feeding parasites that attach to fish, shrimp and snails. They are uncommon in aquariums but occasionally arrive on wild-caught fish or plants sourced from natural waterways. An attached leech causes stress, wounds and potential secondary infection.

What to do

• Remove manually — use tweezers to remove any visible leeches. Check fish bodies for attached individuals.
• Salt dip — a brief (2 to 3 minute) salt dip at one tablespoon per litre causes leeches to detach from fish. Do not perform salt dips on scaleless fish or shrimp.
• Quarantine new arrivals — the best prevention. Quarantine wild-caught fish for two weeks and inspect them daily.

Dragonfly and Damselfly Nymphs

How to identify

These are the larval stages of dragonflies and damselflies. They are relatively large (10 to 40 millimetres), brown or green, with six legs, large eyes and — most distinctively — an extendable lower jaw (labium) that shoots forward to capture prey. Dragonfly nymphs are stout and broad; damselfly nymphs are more slender with three leaf-like gills at the tail.

In Singapore, where dragonflies are abundant year-round, these nymphs appear in open-top aquariums and paludariums more frequently than in temperate climates. Adult dragonflies lay eggs on plant surfaces near water.

Threat level: Dangerous — fish predators

Dragonfly nymphs are voracious ambush predators. They eat fish fry, shrimplets, adult dwarf shrimp and even small fish. A single nymph in a nano tank can deplete the shrimp population within days. They are nocturnal hunters, hiding during the day and ambushing prey at night.

What to do

• Remove immediately — capture with tweezers or a small net. Check the entire tank carefully, as there may be more than one.
• Use a lid — preventing adult dragonflies from accessing the tank surface stops egg-laying. In Singapore’s tropical climate, open-top tanks near windows or balconies are especially vulnerable.
• Inspect new plants — nymphs and eggs sometimes arrive on plants sourced from outdoor ponds or natural water bodies.

Scuds (Amphipods)

How to identify

Scuds are small (3 to 10 millimetres) crustaceans that swim on their sides — a very distinctive behaviour. They are laterally compressed (flattened from side to side), pale grey or brown, and look like miniature curved shrimp. They move quickly, darting among plants and substrate.

Threat level: Usually harmless

Scuds are primarily detritivores and algae grazers. In most tanks they are harmless and even beneficial as part of the cleanup crew. However, some species can damage soft-leaved plants and there are occasional reports of scuds nibbling on fish fins at night, though this is rare and controversial.

What to do

In most cases, leave them. Fish eat them readily and their population self-regulates. If you notice plant damage coinciding with a large scud population, reduce their numbers by trapping (a piece of vegetable in a bottle trap overnight) or let fish control the population naturally. Most community fish find scuds irresistible.

Frequently Asked Questions

I found white worms on my aquarium glass. Should I be worried?

Almost certainly not. White worms on the glass are usually detritus worms, which are completely harmless. If the worms are thin, hair-like and wriggling, they are detritus worms. If they are flat, glide smoothly and have a triangular head, they may be planaria, which require treatment in shrimp tanks. Examine them closely before taking action.

Can I prevent all hitchhikers from entering my tank?

You can reduce the risk significantly by quarantining new plants and livestock, dipping plants in a mild bleach or potassium permanganate solution, and using tissue-cultured plants when possible. However, some microscopic organisms will inevitably enter the tank. The majority are harmless or beneficial. A healthy aquarium has a diverse microfauna — that is a good thing.

Are any of these pests dangerous to humans?

No. None of the common aquarium hitchhikers listed in this guide pose any risk to human health. Leeches in tropical aquariums are freshwater species that very rarely bite humans, and even if they did, the bite is harmless. Wash your hands after working in the aquarium as a matter of general hygiene.

My shrimp are disappearing but I cannot see any pests. What could it be?

Planaria and dragonfly nymphs are both nocturnal and expert hiders. Check the tank with a torch after lights out. Planaria are often found on the glass at night. Dragonfly nymphs hide in substrate and hardscape crevices during the day. Also consider water quality — shrimp are sensitive to ammonia, copper and chloramine in Singapore’s PUB water. Test your parameters before assuming a predator is responsible.

Need Help Identifying Something in Your Tank?

If you have spotted an unfamiliar organism and are unsure whether it is harmless or harmful, bring a photo (or even a sample in a small jar) to our shop at 5 Everton Park. We have been identifying aquarium hitchhikers for over twenty years and can tell you exactly what you are dealing with and how to address it.

Get in touch — we are always happy to help you identify mystery tank inhabitants.

Related Reading

• How to Deal With Pest Snails in Your Aquarium: Prevention and Removal
• How to Deal With Pest Snail Eggs in Your Aquarium
• How to Remove Pest Snails From Your Aquarium
• How to Tell if a Fish Is Male or Female
• Aquarium as Home Décor: Placement, Styles and Design Tips