The nerite snails are tidal snails they are known to bury themselves in the substrate and will even leave the water. Mine will sit an inches out of the water for a good portion of the day some days. Something eatable must be growing under the filter where they are staying. They should eventually spread and eat the algae in the tank. I had 3 in a 30 gallon tank pick it clean. If 3 cant keep up in your tank you may want to reduce the amount of time the light is on to reduce algae growth.