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Aquarium guides/White worms in the tank

White worms in the tank: detritus worms and what they mean

Thin white or pinkish worms wriggling in the gravel or swimming in the water are usually detritus worms, small segmented worms that eat waste and leftover food. They do not harm fish or shrimp. A sudden swarm means there is more food and waste than the tank uses: feed less, vacuum the gravel and change water, and their numbers drop on their own.

Cleaning inside a tank by hand
Shown: cleaning inside a tank by hand. Photo: U.S. Fish and Wildlife Service, Public domain, via Wikimedia Commons

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Detritus worm or planaria?

Detritus worms are thread-thin, segmented and wriggle in S-shapes; many live in the substrate and come out when oxygen runs low. Planaria are flat, glide smoothly along the glass like a tiny slug and have a triangular head; they are the ones to worry about in a shrimp tank.

Thin red threads hanging from a fish’s vent are something else: Camallanus, a roundworm that lives inside the fish and that the Merck Veterinary Manual calls red worm disease. It spreads to other fish and needs a dewormer, so ask a fish vet; cleaning the tank will not clear it.

Why they suddenly appear

  • Overfeeding: food left after two minutes sinks and feeds them.
  • A dirty substrate or a filter long overdue for cleaning.
  • Low oxygen, which drives them up out of the gravel into the water: a sign to check the filter flow and the surface movement.

Bring them down

  1. Feed less, only what the fish finish in about two minutes. How often to feed.
  2. Vacuum the gravel and change 25 to 30 percent of the water, then keep a weekly routine. Cleaning and water changes.
  3. Rinse the filter media in old tank water. Filter maintenance.
  4. Test ammonia and nitrite: a waste problem big enough to feed a worm boom can show there first. What your readings mean.

Many fish eat them, so they rarely last long in a stocked tank once the extra food stops.

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