It was only one of them and they take turns getting on that same rock eating the algae off of it. The rock was covered in it now its looking great because of them lol. They go in and out of the water all the time, I think it's because of the way my tank is setup. Tested water and it came back good.if all your nerites leave the water, it's a good sign that your nitrate levels have spiked, or some other imbalance they don't like.