Neritina snails - best algae eaters?

This photo shows how effective this snail is in eating Green Spot algae as well as the Green Beard algae. One can see the part of the driftwood behind the snail, completely cleaned. The whole wood was covered with this carpet looking algae. I must say that they eat algae very fast. Two snails, that where for last 5 days on a huge rock, cleaned it off of this type of algae. I believe they have some sort of sharp and strong teeth, since they are able to scrape off even the hardest algae that infested the back glass.
I introduced 3 more Neritinas today.