Baby Snails!

[the_lock_man]

THey normally hitchhike in on plants. I keep assassin snails to get rid of any when I add plants.

 

[snazy]

Yes, as lock_man says, they are hitchhiker snails and not nerite snails offspring. There's really more benefits to having snails, than just worry about on the basis of ugly looks.  If you think they are multiplying too fast and you've got too many already, then you are overfeeding or there's decaying matter in the tank. Sorting out those issues by feeding less, cleaning and water changing more will eventually naturally reduce the amount of snails you can see around the tank. Snail overpopulation is like a red flashing sign for the fish keeper that their tactics in balancing the tank aren't right, so I reckon they are a good thing, along with their ability to eat decaying food and algae, thus cleaning and making the tank healthier for the rest of inhabitants and plants, if any.