The water louse: a filter, but much smarter?

Water lice in aquariums: a mobile “filter”… and above all one that chooses its waste!

A classic filter sucks without distinguishing the living from the dead. The water louse, on the other hand, sorts, moving towards waste because it feeds on decomposing organic matter (dead leaves, micro-debris, biofilm). Result: targeted cleaning, much closer to what happens in a pond or natural basin. 

 

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In short

➡️ The water louse (Asellus aquaticus) is a detritivore: it consumes what is dead and/or decomposing, and does not attack living tissues.

➡️ It is active all year round, including at the bottom of frozen waters.

➡️ Experimental results in fish farming show that it can remove dead eggs without attacking viable embryos. Killiphiles use it for this purpose, as it even cleans viable eggs!

➡️ In an aquarium, it settles if you provide it with shelters + supports (mosses, roots, dense areas) and if the fish are not big predators.

 

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The “classic” filter: a blind vacuum cleaner

A filter retains particles and hosts bacteria that degrade part of the organic load. But in its logic, it “takes everything that passes”: waste, microfauna, small fry, etc. And above all, it does not sort.

Besides acting indiscriminately, without choosing what should be preserved, it creates a current, which "degasses" CO2 and thus deprives plants of it. But how else to do it?

 

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The water louse: the “filter” that goes to the waste

Introducing water lice and detritivores into an aquarium or pond is the opposite logic: instead of blindly sucking up everything drifting by (even if it’s alive!), you add a small detritivore that moves itself towards areas where organic waste accumulates.

Specifically, the water louse spends its time:

🍽️ grazing on biofilm and periphyton, composed of bacteria and young algae,

🍽️ nibble on dead tissues and clear them from plants and soil,

🍽️ transform large debris (leaves, remains) into elements easier to process by the microbial chain.

 

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What water lice eat…

They mainly eat

🍃 dead leaves and decomposing plant matter,

🚮 micro organic debris,

🦠 biofilm and associated organisms. 

In very rare cases, and only if they find nothing else to eat, water lice have been observed nibbling on elodea leaves. They are therefore much more respectful of plants than snails.

Their natural cleaning role in water bodies makes them well-known allies and highly sought after for ponds, where they consume and eliminate dead leaves. Some enthusiasts even introduce them into their pond filter, where they work year-round!

Daphnia, ostracods, shrimp, fry, and your fish have nothing to fear from them.

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Water lice and egg clusters: a study that confirms what killiphiles already knew

The idea that water lice can “watch over” a fish spawn comes from a simple observation: they spend a lot of time consuming impurities, and can remove dead eggs that rot.

On this point, an experimental study conducted on Brachydanio rerio reports that A. aquaticus:

✅ eliminates a significant portion of dead eggs (thus protecting healthy eggs from contamination),

✅ distinguishes dead eggs from eggs containing live embryos,

✅ shows no predatory behavior on young fry, even without other food sources.

➡️ Killiphiles have always known this, since they introduce water lice to watch over the spawn: they eat the eggs that rot and even clean the others!

 

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How to install them?

The ideal decor

- Plenty of shelters: mosses, roots, dense foliage, shaded areas.

- Surfaces that naturally develop biofilm: stones, wood, plants, dead leaves, alder cones.

Water lice avoid light and are mainly active at night: a tank rich in shelters makes a big difference.

Temperature and water

They are said to be tolerant over a wide range of temperatures, and also adapted to very cool environments (pond, trash garden, mini-pond). Slightly insensitive to usual water parameters, they seem to be somewhat more numerous in moderately hard to hard waters.
What they fear is the absence of oxygen, such as in a tank or a trash garden exposed to the sun during a heatwave.

Compatibility with fish

🐟 With medium-sized or non-predatory fish: generally simple cohabitation.

🐟 With large predators (goldfish, carp, etc.): they can be eaten; in this case, we rely on lots of hiding places and accept that they can also become natural food.

 

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Why do they have their place in low-tech natural aquariophily?

Because a stable natural aquarium is not just a matter of bacteria: it is a chain.

Water lice add:

👍 a very localized mechanical + biological recycling (they work where it accumulates),

👍 a robust microfauna that participates in the discreet “cleaning”,

👍 a fascinating behavior to observe, especially in natural tanks, trash aquariums, and ponds.

➡️ If your goal is a more “natural” tank, more stable, with fewer wastes settling in dead zones, water lice are among the most consistent helpers.
They complete the chain between bacteria and microfauna on one hand, and snails on the other.

 

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An awareness still emerging

It is estimated that less than 1% of aquarists know about water lice and their role. Still unavailable in stores, they are highly sought after by knowledgeable enthusiasts, whether killiphiles, pond lovers, or "adventurers" of trash aquariums!

But once you've seen them scurry, mate, and fight in an aquarium, you can't do without them!

 

To learn more about "bug aquariums": The phenomenon of fishless aquariums