Are there mud-eating fish for ponds? Tench, carp, snails: a biologist's take

“Put in a tench; it will eat the mud.” This saying has been circulating for generations in garden centers and on forums. It has endured because it is appealing: a living, free solution that works on its own. Unfortunately, no fish eats mud in the way people hope, and most produce more of it than they remove. Here is what the animals known as “cleaners” really do, and what actually reduces the buildup on the bottom of a pond.

First: what is mud?

The mud in a pond is a mixture of decomposing organic matter (leaves, dead algae, waste, and food scraps) and mineral particles (sand, clay, and dust). The organic matter is not food for fish: it has already been largely digested, is nutrient-poor, and microorganisms are responsible for breaking it down. The mineral part, meanwhile, is eaten by no one.

A fish that “forages through the mud” is therefore not looking for the mud itself, but for what lives in it: insect larvae, worms, and small crustaceans.

The tench: the legend of the “pond doctor”

The tench (Tinca tinca) is a hardy bottom-dwelling fish that tolerates low-oxygen water and spends its time foraging through sediment. Its nickname, “doctor,” comes from an old belief that its mucus could heal other fish. This has never been proven.

While foraging, tench stir up the mud and resuspend it. They eat invertebrates, not sediment. And like all fish, they release waste that settles on the bottom. In a garden pond, they are mostly inconspicuous: they are rarely seen, and they do not change the thickness of the mud.

Carp and koi: pond diggers

Carp, including koi, search through the bottom by sucking up sediment and spitting out what they cannot eat. The result is often cloudy water in ponds with soft bottoms, as well as uprooted plants. Koi are heavy consumers of food and heavy producers of waste: in a poorly filtered pond, they create the sludge rather than remove it.

Sturgeon: a fish that fouls the water

Sturgeon searches for food on the bottom with its barbels. It does not consume sludge and requires a rich diet, which means a lot of waste. It also needs highly oxygenated water and plenty of space: it is not a fish for a small pond.

Water snails: helpers, not vacuums

Pond snails, ramshorn snails and river snails graze on algae on the sides and consume some fine plant debris. They contribute to recycling organic matter, but on a modest scale, and their droppings remain on the bottom. In reasonable numbers, they are useful; they will never reduce a visible layer of sludge.

Freshwater mussels: beware

Duck mussels filter the water and retain suspended particles. But they need a soft bottom, plankton-rich water and a population of host fish to reproduce. In a filtered, clear garden pond, they often starve, and a dead mussel quickly pollutes the water.

What really reduces sludge

The only true “sludge eaters” are microscopic. Heterotrophic bacteria break down organic matter into water, carbon dioxide and minerals. They must be present in sufficient numbers and have enough oxygen. An effective strategy combines three actions:

1. Add specialized bacteria from April to October, when the water is above 10 to 12 °C.
2. Oxygenate the bottom with an air pump and diffuser so decomposition is fast and odor-free.
3. Remove heavy deposits mechanically with a pond vacuum, then let the bacteria maintain it.

What if I still want bottom-dwelling fish?

There is nothing to prohibit it, provided it is done for the right reasons. One or two tench in a sufficiently large natural pond fit in well and consume larvae. In a small garden pond, it is better to abstain: each additional fish increases the organic load. The same rule applies to all species: do not overstock or overfeed.

Read also

Sludge in the pond: vacuum or bacteria, which should you choose?

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Frequently asked questions

— Do catfish eat sludge?

No. They feed on invertebrates, leftovers, and small fish. Some, such as the American catfish, are also considered invasive and must not be introduced.

— Do crayfish clean the bottom?

They consume debris, but dig into banks and can damage a liner. Several species are invasive, and their introduction is regulated.

— How many tench for a pond?

In a garden pond, none are necessary. In a natural pond of several dozen cubic metres, one or two are sufficient.

— Do bacteria replace a vacuum?

They maintain and break down recent deposits. When faced with a thick layer, one vacuuming saves a season, then the bacteria take over.

In summary

• No fish eats sludge; bottom-feeders stir it up.
• Every fish adds waste, and therefore sludge.
• The real decomposers are bacteria, with oxygen.
• Winning strategy: targeted suction, bacteria, aeration.

Last updated: 28 September 2026.