Do snails clean up ponds?

Neil Phillips.

Great Pond Snail (Lymnaea stagnalis) from below. Copyright: Neil Phillips.

People writing about ponds often repeat the idea that ‘It is worth acquiring water snails from a pond supplier because they help to keep the pond clean…‘.

But is this appealing idea true? Unfortunately not.

So what’s really going on?

As everyone who’s kept snails in an aquarium knows, snails graze algae, the algae that grow on everything under the water: plants, stones, fallen leaves, dead wood, the glass of a tank or the sides of a pond.

These algae are epiphytes: just like mosses and lichens on the trunks of trees, they are small plants that grow on bigger plants. They are a perfectly natural part of the pond environment.

They are also the food of many pondy animals, like snails, mayflies and caddis flies, which graze them just as cows in a field graze on grass.

But like all algae in ponds, if you add too much fertiliser to the water, they go mad – and just grow and grow, beginning to smother everything.

When people talk of keeping a pond clean, what they usually mean is removing these excess algae, which are the result of the pond being polluted by nutrients – from tap water, added top soil or added fish food.

Snails will eat some of this surfeit of algae – but they can’t do anything to cure the cause of the problem, which is the over-fertilised pond – and I’ve never seen a pond where they actually controlled the algae.

Its nice to have snails – they are a natural part of the community of ponds that have enough calcium to allow their shells to grow – but value them because they are a natural part of the pond’s wildlife.

To keep your pond clean, have clean water to start with, and nix the fertilisers.

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