Little Worms in Your Saltwater Tank: What They Are and What to Do About Them

In this guide

Spot a few thin, pale threads wriggling along the glass or poking out of the sand and the first instinct is to panic. Take a breath. Most of the tiny worms that turn up in a saltwater tank are harmless, and a few are even doing you a favour.

Detritus worms

These are the most common find. They are thin, segmented worms, white, grey or pale brown, and closely related to earthworms. They live in the sand bed and rock, feeding on leftover food and decaying waste. A moderate population is actually a sign that your tank has a healthy clean up crew working the substrate. Numbers only become a problem when they explode, and that almost always points back to overfeeding or a substrate that has not been disturbed in a long time.

Spirorbid worms

Look closely at the glass or rockwork and you may see tiny white spiral shells, barely a couple of millimetres across. These are spirorbid tube worms, harmless filter feeders that arrive on live rock or sand. They cannot hurt fish or corals, though a heavy crust of them on intake pipes or powerhead impellers can eventually clog equipment, so an occasional scrape during maintenance is worth doing.

Bristleworms and copepods

Bristleworms are covered in more detail elsewhere in this series, since they deserve their own explanation of the harmless scavenger versus the more aggressive fireworm. Copepods and amphipods, the tiny white specks that dart around the glass at night, are not worms at all but crustaceans, and they are firmly on the beneficial side of the ledger, forming a natural food source for mandarins and other fussy eaters.

When to actually worry

The rare genuinely harmful worm in a marine tank is a parasitic species attached to a fish itself, visible as a thread trailing from the skin or gills, usually accompanied by the fish scratching against rock or glass. That calls for a proper quarantine and treatment plan, not a generic worm remedy.

Keeping numbers sensible

• Cut back on feeding if you are seeing worms in large numbers, since more food waste simply means more worms.
• Vacuum a portion of the sand bed during water changes to physically remove excess detritus and worm populations along with it.
• Add a wrasse or other worm eating fish only if it suits your existing stock, rather than as the first line of defence.
• Resist reaching for a dewormer unless you have positively identified a parasitic species, since broad chemical treatments can wipe out beneficial copepods and amphipods along with the problem.

Because most live rock and live sand sold in India is imported or transhipped through Southeast Asian and Sri Lankan suppliers, a fresh consignment often carries a noticeable bloom of these hitchhiker worms in the first few weeks. Give it time before assuming something has gone wrong.

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