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What is it[edit]

A nematode is a micro sized worm often called roundworm (they are roughly 2-5mm long). There are over 20,000 species of them. and they provide an important service in ponds, lakes, etc. They live in all habitats, and most are aquatic. They generally are only a few mm in length and have a life-cycle of a few days.

  • Some species are parasitic and live off fish, often appearing in the anus of the fish. But by far the majority are harmless.


Aquarists often first come across these harmless 'white worms' when they spot them in their hundreds massing on the sides of the aquarium in newly set up aquariums during the cycle process, when the aquarium is lacking in oxygen or if the tank is heated. They help to break down organic matter (mulm) and some fish like small gouramis are know to eat them.


If you've added something organic (wood, other fish,raw fish food,etc.) to the tank you can easily introduce them. Leaving uneaten pet food in the water will virtually guarantee an outbreak of them.

Getting rid of them[edit]

  • Simply reduce the amount you feed your pets.
  • Siphon up uneaten food.
  • Add species of fish like Gourami or Pleco which will eat them.

They often simply disappear after 3 or 4 days if it's a newly cycled tank.

If they appear to be coming from the anus area of fish then more drastic deworming medicine may be required.


If they look kind of flat and white, then they're most likely Planaria flatworms. They are not harmless and can infest the gills of your fish leading to an early death. A deworming medicine cycle is needed.