Snails forage on left over food and graze on algae therefore a small colony should not be of concern. In fact, they are doing you a favour by eating excess food (and can be amusing to watch and even add to a more "natural" look in your aquarium!). Some species also help by burrowing through the substrate thus preventing compacting and dead spots and even to help dispose of dead fish. However, as with all living creatures, snails produce excreta and thus large colonies of snails can result in quickly deteriorating water quality. Some species do damage plants, and large numbers may look unsightly.
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