Isopods in the aquarium: what they are, what they look like, where they come from and should you be worried

What are isopods?

Isopods are small crustaceans (related to shrimp and pill bugs). In freshwater aquariums you usually see aquatic isopods: flattened, oval or elongated, often grey, brown, or translucent, with several pairs of legs. They eat decaying matter, biofilm, and sometimes leftover food.

What do they look like?

Typically 2–10 mm, flat or slightly rounded, with a segmented body and visible legs. Colour ranges from white/grey to brown. You might spot them on the glass, in the substrate, under decor, or on plants. They are often more active at night.

Where do they come from?

Almost always hitchhikers: on live plants, in damp substrate or decor from another tank, or in water from a store. They are very good at hiding in plant roots or folds.

Should you be worried?

Usually no. They do not attack healthy fish or snails. They are part of the cleanup crew. Some fish will eat them. If numbers explode, it often means excess food or organic waste — reduce feeding and maintenance. If you do not want them, remove by hand and avoid introducing more on new plants (rinse and inspect). They are not harmful.

Quick takeaways

• Isopods are small crustaceans; grey/brown, flat, segmented, often on glass or in substrate.
• They usually hitchhike in on plants, decor, or from another tank.
• They are not a danger to fish; reduce numbers by less feeding and good maintenance if you want fewer.