What are these snails and what do they do?

Donya said:

These are not assasin snails (Clea helena), but rather a species of Malaysian Trumpet Snail, which is the common name for several species in the family Thiariidae. My guess would be Tarebia granifera.

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Are they harmless or a pest?

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The Malaysian Trumpet Snail is actually a benefit to any aquarium for several reasons. It will not eat your plants at all. This snail also will not "muscle" your shrimp off of food meant for the shrimp. The Malaysian Trumpet Snail feeds on detritus and leftover food that is underneath the substrate. It actually burrows in the substrate and moves around throughout. You will rarely see this snail during the day. Occasionally it will emerge from the substrate. A cool thing is that sometimes you will see the substrate move and you will know that there is a Malaysian Trumpet Snail underneath doing its cleaning duties.

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